Chromis Care Guide: Schooling Reality, Tank Size and Best Chromis for Reefs

Introduction

Chromis are among the most commonly recommended schooling fish for reef aquariums, and it’s easy to see why. These active, colorful fish hover in the water column, adding movement and life to any reef tank. However, there’s a persistent misconception that chromis will behave like freshwater schooling fish in captivity, swimming in tight synchronized formations across your tank. The reality is quite different.

While chromis form large aggregations on coral reefs in the wild, aquarium environments often cause very different social behaviors. Limited space triggers dominance hierarchies, and what starts as a peaceful group can gradually become a single fish ruling the tank.

This guide covers what actually happens when you keep chromis in reef tanks, including species differences, tank size requirements, feeding strategies, and practical approaches to keeping these fish successfully long term. Whether you’re planning your first chromis purchase or trying to figure out why your group keeps shrinking, you’ll find real answers here.

What Are Chromis Fish?

Chromis are small reef fish belonging to the damselfish family (Pomacentridae). If you’ve kept damselfish before, you might expect chromis to be aggressive little terrors, but they’re actually quite different from their more territorial cousins. Chromis are a well known member of the damselfish family that evolved for open-water living rather than defending specific coral heads. Blue Green Chromis are hardy and relatively easy to care for, making them suitable for beginners.

Unlike other damselfish that stake out territory and fight anything that comes close, chromis spend most of their time swimming in the water column above reef structures. They’re planktivores, constantly picking tiny zooplankton from the water as currents sweep food past them. In the wild, they form large aggregations numbering in the hundreds or thousands, hovering above branching corals like living clouds.

Key characteristics of chromis:

• Active swimmers that hover in the middle water column

• Planktivorous feeding habits (hungry fish that graze continuously)

• Naturally found in large aggregations on coral reefs

• Generally considered reef safe with corals and invertebrates

• More peaceful temperament than other damselfish species

Chromis became popular in the reef aquarium hobby back in the 1980s when miniature reef systems started gaining traction. Their hardiness, vibrant coloration, and active swimming made them seem like perfect “filler” fish to add movement and color. With proper care and sourcing, chromis can remain alive and thrive for many years in captivity. Blue-Green Chromis are also compatible with other peaceful reef fish like blennies, clownfish, and gobies. All of this contributed to chromis becoming a staple recommendation for new reef hobbyists.

Chromis Behavior in the Wild vs Reef Aquariums

Understanding how chromis behave in nature helps explain why they often act so differently in our tanks. The behavioral shift between wild and captivity isn’t random, it’s a predictable response to environmental constraints.

In aquariums, group attrition is common, with hobbyists frequently noticing that their school of chromis gradually shrinks over time. Hierarchical aggression is a primary factor in Green Chromis losses in captivity, often resulting in subordinate fish that have died due to stress or aggression from dominant individuals.

Wild Schooling Behavior

On Indo-Pacific coral reefs, chromis viridis and other species form massive aggregations that hover above branching coral formations. These aren’t tight schools like you’d see with sardines or anchovies. Instead, chromis form loose clouds that shift and flow with currents, individual fish constantly moving while maintaining general proximity to the group.

This aggregation behavior serves two main purposes. First, feeding efficiency, zooplankton concentrations drift through reef currents, and by spreading out in the water column, each fish maximizes its chances of intercepting food. Second, predator avoidance, there’s safety in numbers, and reef predators have a harder time targeting individual fish within a large shoal. Forming large aggregations helps provide safety for chromis by making it more difficult for predators to single out and capture any one fish.

Factors supporting natural schooling behavior:

• Vast swimming space measured in thousands of gallons

• Constant predator pressure maintaining group cohesion

• Large group sizes (often hundreds of fish)

• Natural escape routes within complex coral structures

• Continuous food availability from plankton-rich currents

The key observation here is that wild chromis don’t school because they enjoy each other’s company. They aggregate because environmental pressures make it advantageous for survival. Remove those pressures, and the behavior changes.

Hierarchy Development in Aquariums

Aquariums fundamentally lack the conditions that support natural chromis schooling. There’s no predator pressure, limited escape routes, restricted territory, and food arrives on your schedule rather than drifting continuously past. In this environment, chromis social behavior shifts dramatically.

Within days of introduction to a tank, chromis begin establishing dominance hierarchies. One fish becomes the alpha, claiming the best feeding position in the upper water column. Others fall into a pecking order below. The alpha gets first access to food and the prime swimming territory, while subordinates are pushed to less desirable areas.

Progression of social behavior in captive groups:

• Dominant individuals establish territories and prime feeding positions

• Subordinate fish experience increased stress and reduced food intake

• Aggression manifests as fin nipping, chasing, and exclusion from feeding

• Weaker fish become emaciated, stressed, and susceptible to disease

• Gradual loss of group members over weeks to months

Overcrowding in an aquarium can contribute to territorial disputes and stress among Green Chromis, leading to health issues.

This process is so common that reef hobbyists have a term for it: group attrition. You buy six green chromis, and six months later you have two. A year later, maybe one. It’s frustrating, but it’s predictable behavior once you understand the underlying social dynamics.

Subordinate chromis often seek out hiding places to escape aggression. Providing hiding spots or small caves in the aquascape allows chromis to rest and recover from social stress, which is important for their health and well-being.

Do Chromis Actually School in Aquariums?

Here’s the direct answer to the question everyone asks: chromis typically form loose groups rather than tight schools in aquariums, and even these loose associations often break down over time due to hierarchy development.

True schooling, synchronized movement, tight formation, rapid coordinated turns, requires conditions home aquariums rarely provide. You’d need massive water volume, constant water movement with plankton drift, and group sizes large enough to overwhelm individual territorial instincts. Some public aquariums achieve this with displays of 50+ chromis in multi-thousand-gallon systems. Most aren’t working with those parameters.

Factors that influence group stability in home aquariums:

• Tank size (larger tanks dilute aggression through spatial separation)

• Group size (very small or very large groups work better than medium)

• Aquascaping layout (open swimming space vs cluttered rockwork)

• Importance of rocks in aquascaping to provide territory boundaries and hiding spots

• Availability of hiding spaces and territory markers

• Feeding frequency (more frequent feeding reduces competition stress)

That said, some hobbyists do achieve stable chromis groups. The success stories typically involve large systems of 100+ gallons with open aquascaping, groups of 8-12 fish, and consistent husbandry practices. Even in these setups, expect some initial attrition as hierarchy stabilizes. The goal isn’t eliminating hierarchy behavior, it’s creating conditions where that hierarchy doesn’t result in fish dying.

Blue Green Chromis should be kept in a small school of 3 to 5 individuals. A larger tank is recommended for keeping Blue Green Chromis in schools to provide ample swimming space and reduce territorial disputes.

Best Chromis Species for Reef Aquariums

Several chromis species show up regularly in the aquarium trade. While they share general care requirements, behavioral differences between species influence how you should approach stocking and tank planning. Blue Green Chromis are active swimmers and require a tank setup that allows for swimming space in the middle to top areas of the tank. They thrive in stable water conditions and require consistent care to maintain their health. During spawning, chromis may display color changes and nest-building behaviors.

Green Chromis (Chromis viridis)

The green chromis, also called blue green chromis due to its shifting iridescent coloration, is by far the most common chromis in reef aquariums. Walk into any local fish store (LFS) and you’ll likely find them in the fish tanks, often sold in groups specifically for schooling displays.

Key details:

• Adult size typically 3–4 inches

• Bright green to blue-green iridescence that shifts with lighting

• Indo-Pacific origin (collected from Indonesia, Philippines, and similar regions)

• Often sold in groups for schooling displays

• Lifespan of 8-15 years with proper care

In breeding, adults play distinct roles: males prepare nests and fertilize eggs, while females lay eggs during spawning. After hatching, fry require a diet high in fat and protein to support proper development.

Green chromis are widely recommended for reef aquariums because they’re hardy, affordable, and adapt well to captive conditions. However, they’re also the species most commonly misunderstood regarding schooling behavior. New hobbyists buy groups expecting synchronized schools and end up watching gradual attrition instead.

One advantage of green chromis is their tolerance for water quality fluctuations. They handle nitrates up to 20ppm reasonably well, which gives hobbyists more margin for error than some sensitive species. Their iridescent coloration actually intensifies with vitamin-enriched feeding, a detail worth noting for maintaining that vibrant blue-green appearance.

For optimal health, Blue-Green Chromis require water temperatures of 72–82°F (22–28°C), a pH of 8.1–8.4, and a salinity of 1.020–1.025 SG. Performing regular water changes (10-25% weekly or fortnightly) is necessary to maintain optimal water quality.

Blue Reef Chromis (Chromis cyanea)

Blue reef chromis come from Caribbean waters rather than the Indo-Pacific, and they bring a slightly different temperament to the tank. Their coloration is a deeper, more uniform blue compared to the green chromis’s shifting iridescence.

Key characteristics:

• Slightly more assertive temperament than green chromis

• Adult size around 4–5 inches (larger than green chromis)

• Native to Caribbean reef environments

• Requires higher flow and larger tank systems

• Less tolerant of elevated nitrates (prefer under 10ppm)

In my observation, blue reef chromis tend to develop stronger territorial behavior than their Pacific cousins. They need larger tanks, 100+ gallons for groups, and don’t mix well with green chromis in the same system. I’ve seen hierarchy clashes occur between species that result in worse aggression than either species shows alone. If you’re planning chromis, pick one species and stick with it.

Black Axil Chromis (Chromis atripectoralis)

Black axil chromis are sometimes mistaken for green chromis, but they’re a distinct Pacific species with different behavioral tendencies. The identifying feature is a dark marking near the pectoral fin base, hence “black axil.”

Key characteristics:

• Slightly larger adult size than green chromis

• Dark marking near pectoral fin base (diagnostic feature)

• Can show stronger territorial behavior in aquariums

• Tolerates smaller group sizes (4-6 in 50 gallons)

• Shows more interest in grazing algae than other chromis

Black axil chromis seem to tolerate smaller groups better than green chromis, making them potentially more suitable for medium-sized systems where you can’t maintain large groups. However, they can also show more pronounced territorial behavior when hierarchies form. Research your source population, as different collection regions may show different behavioral tendencies.

How Many Chromis Should You Keep?

Stocking strategy might be the single most important decision when keeping chromis. Get this wrong, and you’ll watch your fish disappear one by one. Get it right, and you’ll enjoy chromis for years.

Single Chromis

Keeping a single chromis eliminates hierarchy problems entirely. Without other chromis to dominate or be dominated by, individual fish develop normal activity patterns, show consistent coloration, and integrate easily with other tank mates.

Benefits of single chromis:

• No hierarchy conflicts or aggression

• Consistent feeding without competition

• Easy integration with other community fish

• Predictable, stable long-term success

• Full personality development without social suppression

Recommended tank size typically starts around 30–40 gallons for a single specimen. The fish needs adequate swimming space in the water column but doesn’t require the territory dilution that groups demand. Sometimes a single green chromis can thrive for 10+ years in 40-gallon reef tanks, showing all the activity and color that makes the species attractive without any of the group dynamics headaches.

If your goal is simply adding an active swimmer to your reef community, a single chromis delivers great results with minimal risk.

Small Groups (3–7 Chromis)

Small groups represent the most common approach in medium-sized reef aquariums—and unfortunately, the approach most likely to result in problems. This middle-ground stocking creates conditions where hierarchy develops but doesn’t have enough fish to distribute aggression effectively.

Considerations for small groups:

• Hierarchy formation is essentially guaranteed

• Need for adequate swimming space becomes critical

• Increased feeding requirements to reduce competition stress

• Odd numbers may help prevent pairing off and ganging up

• Groups may gradually shrink if aggression develops

In tanks of 55-75 gallons, small groups of 4-6 chromis can frequently reduce to 2-3 fish within a year. Sometimes a stable pair or trio forms, but often the dominant fish eventually picks off all competition. If you choose this approach, plan for potential attrition and be prepared to rehome aggressive individuals or transition to single-fish keeping.

Large Groups (8+ Chromis)

Large groups in appropriately sized tanks offer the best chance of stable chromis keeping. The principle is simple: with enough fish, no single individual can dominate everyone, and aggression becomes diluted across the group.

Advantages of large groups:

• More natural aggregation behavior

• Reduced focus on any single subordinate fish

• Greater likelihood of stable hierarchy formation

• Visual impact of multiple active swimmers

• Better approximation of wild social dynamics

Recommended tank sizes typically exceed 100 gallons for groups of 8-12 chromis. These systems need substantial open swimming space, at least 40-60% of the upper tank volume without dense rockwork blocking movement. High flow from wavemakers (20-40x turnover) helps simulate reef currents and encourages natural feeding behavior.

Introducing all fish simultaneously as juveniles gives the best results. Adding chromis to an established group invites immediate aggression toward newcomers. Buy your group together, quarantine together, and add together.

Tank Size and Aquascaping Requirements

Tank volume matters for chromis, but how you organize that volume matters just as much. These fish need open swimming space in the water column combined with reef structure that provides security without creating dead-end hiding spots where bullied fish get trapped. Blue Green Chromis can be kept with most other fish in a saltwater aquarium, excluding large predatory species, making them a great choice for community tanks with compatible tank mates.

Aquascaping approaches that work well for chromis:

• Branching coral structures (live or decorative) positioned to create retreat areas

• Rock formations providing visual barriers between territories

• Open midwater swimming zones without obstructions

• Lagoon-style layouts with scattered bommies rather than solid walls

• Vertical structure with spaces between rock and glass walls

Avoid dense aquascapes that fill the tank with rockwork. Chromis need swimming lanes through the open water column, not mazes to navigate. Position large rock structures 8-12 inches from tank walls and 12-18 inches from each other to create multiple territorial areas without crowding.

The success rate column reflects long-term keeping (1+ years) based on typical hobbyist outcomes. Small groups in medium tanks can work, but they require more attention to aggression management and feeding distribution. Large systems with appropriate stocking approach 80-90% group retention after the initial hierarchy stabilizes.

Compatible Tankmates

Choosing the right tankmates for your green chromis (Chromis viridis) or blue green chromis is essential for maintaining a peaceful and vibrant reef aquarium. These small fish are well known members of the reef community thanks to their active swimming and schooling behavior, but their success depends on the company they keep.

Green chromis thrive alongside other peaceful species that won’t outcompete them for food or territory. Classic choices include clownfish, yellow tang, and non-aggressive damselfish. These species tend to occupy different areas of the tank, reducing direct competition and allowing your chromis to swim freely in the middle water column. Cleaner shrimp and other invertebrates also make excellent tankmates, as they help maintain a healthy environment and provide natural cleaning services without posing any threat to your chromis.

When keeping chromis in large systems, maintaining a school of at least 6-8 individuals provides safety in numbers and encourages natural schooling behavior. In smaller tanks, individual fish may become more territorial or stressed due to the lack of schooling opportunities, so tank size should always be matched to your stocking plan. Providing plenty of hiding places and visual barriers,such as branching rockwork or coral structures,helps reduce stress and gives each fish a place to retreat if needed.

A varied diet is another key to harmony in your reef aquarium. Feeding a mix of frozen foods, high-quality pellets, and occasional live treats ensures all fish, including your chromis, get the nutrition they need. This reduces competition at feeding time and helps prevent aggressive behavior from hungry fish.

When introducing new tankmates, add them gradually and observe their reactions closely. Even peaceful species can show unexpected aggression if the tank is overcrowded or if food is scarce. Regular water changes and careful maintenance of water quality will help keep all your fish healthy and stress-free.

Feeding Chromis in a Reef Tank

Chromis are hungry fish with metabolisms built for continuous grazing. In the wild, they feed constantly as plankton drifts past on currents. Replicating this feeding pattern, or at least approximating it, supports health, coloration, and social stability.

Natural Diet and Nutritional Needs

Wild chromis feed primarily on zooplankton, the tiny crustaceans and larvae that drift through reef waters. This protein-rich diet powers their active swimming behavior and supports their iridescent coloration. They also pick at filamentous algae and may consume detritus, but plankton forms the core of their nutrition.

In aquariums, chromis need a varied diet that approximates this nutritional profile. Aim for approximately 35-45% marine protein, 25-35% vegetable matter, appropriate omega-3 fatty acids, and some fiber content. Vitamin enrichment noticeably enhances coloration, hobbyists report 20-30% improvement in iridescent blues when feeding enriched foods compared to basic flakes.

Recommended Foods

A good chromis feeding program includes variety across food types and brands. No single food provides everything these fish need, so rotation keeps nutrition balanced and maintains feeding interest.

Examples of suitable aquarium foods:

• Frozen mysis shrimp (primary protein source)

• Copepods and plankton blends (closest to natural diet)

• High-quality marine pellets (1-2mm size, New Life Spectrum or similar)

• Enriched brine shrimp (supplemental, not primary)

• Spirulina and marine algae preparations (vegetable component)

Pellets work well for routine feeding because they’re consistent and easy to portion. Frozen foods should be offered at least every other day for optimal nutrition. Live copepods, if available, encourage natural feeding behavior and provide nutrition that frozen foods can’t fully replicate.

Feeding Frequency

Frequent small feedings support chromis better than once-daily large meals. Two to three feedings per day, offering only what fish consume in 2-5 minutes, mimics natural grazing patterns and reduces competition stress within groups.

Recommended feeding schedule:

• Two to three small feedings per day

• Small portions consumed quickly (2-5 minutes maximum)

• Multiple feeding locations for groups (broadcast from different points)

• Turn off return pumps during feeding to keep food in feeding zone

Feeding frequency becomes especially important for groups. Dominant fish monopolize surface feeding areas, so subordinates benefit when food reaches multiple tank locations. Use a turkey baster to target-feed skittish individuals if you notice uneven food distribution.

Are Chromis Reef Safe?

Yes, chromis are considered fully reef safe. They don’t nip corals, won’t eat your shrimp or snails, and generally ignore invertebrates entirely. This makes them compatible with virtually any reef aquarium setup.

Compatibility details:

• Do not damage corals (SPS, LPS, or soft corals)

• Safe with cleaner shrimp, ornamental shrimp, snails, and crabs

• Generally compatible with peaceful reef fish

• Mid-water swimming position minimizes territorial conflicts with bottom dwellers

Chromis occupy a specific niche in reef tanks, the open water column. They don’t compete for territory with gobies hiding in burrows, clownfish hosting in anemones, or tangs patrolling the rockwork. This spatial separation reduces conflict with other species.

Compatible tankmates for chromis:

• Clownfish (pair well in most community reefs)

• Gobies and blennies

• Firefish

• Fairy wrasses

• Cardinalfish

• Yellow tang and other peaceful tangs

Avoid mixing chromis with aggressive damselfish species, as territorial damsels will harass chromis constantly. Also avoid keeping chromis with predatory fish large enough to eat them, triggers, groupers, and large wrasses view small fish as potential food rather than tankmates.

One notable consideration: chromis may compete with anthias for planktonic food in the water column. Both species occupy similar niches, and adding anthias to an established chromis group (or vice versa) can disrupt feeding hierarchies. Plan your stocking order carefully if you want both.

Common Problems When Keeping Chromis

Even with proper care, chromis present specific challenges. Understanding these problems before they occur helps you respond effectively, or prevent them entirely.

Group Attrition

The most common problem with chromis is watching your group shrink over time. You start with six fish and end up with one. This happens because hierarchy stress causes subordinate fish to eat less, hide more, and become vulnerable to disease. Eventually they die or get killed outright by dominant individuals.

Solutions for group attrition:

• Keep larger groups in bigger tanks (dilutes aggression)

• Remove overtly aggressive individuals immediately when identified

• Transition to single chromis keeping if attrition continues

• Ensure multiple feeding locations so subordinates access food

• Provide adequate territory separation through aquascaping

If you’re already experiencing attrition, the best response depends on how many fish remain. Groups of 3-4 survivors often stabilize, especially in tanks 75+ gallons. If you’re down to 2 fish and one is clearly dominant, consider removing the aggressor or accepting that you’ll eventually have a single chromis.

Disease Susceptibility

Chromis under social stress show weakened immune systems and increased susceptibility to common reef diseases. Ich (marine white spot disease) is particularly common in stressed chromis, along with bacterial infections that affect fins and body tissue.

Preventative strategies:

• Quarantine all new fish for 4 weeks before main tank introduction

• Maintain stable water parameters (temperature 75-82°F, pH 8.1-8.4, salinity 1.020-1.025)

• Provide proper nutrition with varied, enriched foods

• Reduce hierarchy stress through appropriate stocking

• Remove severely stressed fish to hospital tank before disease advances

Treat ich and bacterial infections in copper-free hospital tanks rather than main reef displays. Copper medications kill invertebrates and can damage coral tissue, making them unsuitable for reef aquariums even at “safe” concentrations.

Aggression Within Groups

Even in well-planned setups, individual chromis sometimes become problematic bullies. One fish may relentlessly pursue others, claiming the entire tank as territory rather than sharing space.

Management strategies:

• Remove the aggressive individual and observe if behavior transfers to next-dominant fish

• Rearrange rockwork to reset territorial claims

• Increase feeding frequency to reduce competition

• Add more chromis to overwhelm the bully (only in large tanks with quarantine)

Sometimes removing the alpha just promotes the beta to aggressive dominance. If multiple fish cycle through aggressive behavior, the problem likely relates to tank size or aquascaping rather than individual fish personality. Consider whether your system can actually support the group you’re attempting to keep.

Are Chromis Good Fish for Beginners?

Chromis occupy a complicated position as beginner fish. They’re widely recommended, but the recommendation often comes without realistic expectations about behavior and keeping challenges.

Advantages for beginners:

• Reef safe with corals and invertebrates

• Active swimmers that add visible movement

• Widely available and affordable

• Hardy and adaptable to aquarium conditions

• Tolerate parameter fluctuations better than many reef fish

Potential challenges for beginners:

• Schooling expectations rarely match reality

• Social hierarchies can cause frustrating group instability

• Multiple feedings daily required for optimal health

• Success requires understanding damselfish social behavior

For new reef hobbyists, a single chromis offers the best introduction to the species. You get the active swimming behavior and attractive coloration without the complexity of group dynamics. Once you’ve maintained a reef successfully for a year or more and understand water quality management, you can consider larger systems that support chromis groups.

If you plan to start with a group despite limited experience, research beforehand and set realistic expectations. Accept that some attrition may occur, buy juveniles simultaneously rather than adding over time, and commit to the frequent feeding schedule these fish require.

Conclusion

Chromis success in reef aquariums comes down to understanding their real behavior rather than expecting them to school like freshwater tetras. These fish evolved for specific reef conditions that aquariums rarely replicate, and their social dynamics shift accordingly.

Key takeaways:

• Chromis develop dominance hierarchies in aquariums rather than forming tight schools

• Tank size strongly influences group stability, bigger is better

• Single fish or large groups (8+) tend to produce better outcomes than medium groups

• Frequent feeding supports health, coloration, and reduces competition stress

• Reef compatibility is excellent, but avoid mixing with aggressive species

The choice between keeping a single chromis versus attempting a group should be based on your tank size and commitment level, not assumptions about schooling behavior. A well-cared-for single chromis provides years of enjoyment with minimal complications. Groups require larger systems, more attention, and acceptance of some initial attrition before stabilizing.

Whatever approach you choose, plan your stocking strategy before purchasing. Walk into your LFS with clear expectations about what works for your specific system. Chromis are fine fish that contribute real value to reef aquariums, when kept appropriately.

FAQ

Do chromis actually school in aquariums?

Chromis usually form loose groups rather than tight synchronized schools in aquariums. The space constraints and lack of predator pressure remove the environmental factors that drive schooling behavior in the wild. Some hobbyists achieve more cohesive groupings in very large tanks (150+ gallons) with 10+ fish, but even these are aggregations rather than true schools. Expect chromis to swim in the same general area while maintaining individual spacing rather than moving as a coordinated unit.

How many chromis should be kept together?

The answer depends heavily on your tank size. Single chromis work well in tanks 30-40 gallons and larger, eliminating all hierarchy problems. Groups of 3-7 in medium tanks (55-75 gallons) often experience attrition over time. Large groups of 8-12 in tanks exceeding 100 gallons have the best chance of long-term stability. If choosing groups, add all fish simultaneously as juveniles rather than staggering introductions.

What tank size is best for chromis?

For a single chromis, 30-40 gallons provides adequate swimming space. For stable groups, plan on 75-100+ gallons minimum, with larger tanks producing better outcomes. The critical factor isn’t just volume but open swimming space, a heavily aquascaped 100-gallon tank may support chromis poorly if most volume is occupied by rockwork. Aim for 40-60% open water in the upper half of the tank.

Are green chromis aggressive?

Green chromis are peaceful toward other species but can develop significant aggression toward their own kind. The distinction matters, your chromis won’t harass your clownfish or fight with gobies, but they will establish dominance hierarchies within chromis groups that can result in fin damage, starvation, and death of subordinate individuals. This semi-aggressive behavior toward conspecifics surprises many hobbyists who expect uniformly peaceful fish.