Whitespotted Pygmy Filefish

The Whitespotted Pygmy Filefish (Rudarius ercodes), also called the White-spotted Pygmy Filefish, is a compact marine fish with a pointed snout, a broad rounded tail, and small pale spots. Its ground color can shift between green, olive, and darker shades. The spots may stand out clearly one moment and become much less visible the next, depending on its surroundings and behavior.

This species occurs in the northwestern Pacific around Japan, southern Korea, and Taiwan. It is associated with bottom cover, including eelgrass beds and macroalgae, where it can hover close to vegetation and pick at small food items. Wild diet records include amphipods, worms, and eelgrass, with copepods particularly important for small juveniles.

In an aquarium, the Whitespotted Pygmy Filefish suits a peaceful community with sheltered feeding areas and room to browse around the rockwork. Its small adult size allows a modest setup for one fish, provided larger tankmates leave it access to food. Captive-bred fish of this species are available in the trade, and acclimation conditions should reflect the source of the individual.

Quick Care Guide

Care Level: Moderate
Minimum Tank Size: 20 gallons
Adult Size: About 3 inches (7.5 cm total length)
Temperament: Peaceful
Diet: Omnivore
Reef Safe: With Caution
Region: Northwestern Pacific, including Japan, southern Korea, and Taiwan
Scientific Name: Rudarius ercodes

Tank Requirements

Provide at least 20 gallons for one adult. The species reaches about 3 inches (7.5 cm) in total length. Use a larger aquarium for a group or for a community in which several fish feed around the same rockwork. Choose a footprint that leaves room for the filefish to maneuver after the shelters are in place.

Arrange stable live rock with ledges, crevices, and several open feeding positions just outside cover. Upright structure or established marine macroalgae can provide extra places to hover. Leave a clear area where fine food is visible and accessible, and keep enough space around the rock to remove settled debris.

Provide a choice of flow. This species can tolerate moving water once acclimated, and a quieter area gives it a place to hold position or feed. Direct a gentle current past the main shelter and keep stronger circulation elsewhere for oxygen exchange. Watch how food drifts before adjusting pump direction or adding more food.

Use a cycled aquarium with dependable filtration and fit a cover around plumbing and cables. Protect accessible intakes, especially in a small display. A newly introduced fish may stay close to one corner or ledge. Move its feeding station near the cover it actually uses and give it a second retreat away from tankmates.

The species comes from temperate waters, and aquarium-raised specimens are commonly kept at 75–78°F (24–26°C). Confirm the holding temperature for a newly imported or wild-collected specimen before choosing its long-term setup. Use stable, compatible conditions and avoid sudden temperature changes at introduction.

Diet and Feeding

The Whitespotted Pygmy Filefish is an omnivore that naturally takes small animal prey as well as plant material. Offer a varied rotation of finely sized marine foods, including small crustaceans, complete preparations, and some algae-based food. Cut larger seafood into pieces the fish can swallow easily with its small mouth.

Rotate foods from these options.

• Frozen copepods or other fine plankton foods
• Small frozen mysis shrimp
• Enriched brine shrimp
• Finely chopped marine shrimp or clam
• Balanced frozen marine omnivore foods
• Fine pellets or spirulina-based marine foods once accepted

Start with two to three small feedings per day. Release a few pieces into the gentle current near cover and watch the fish take them. If fast tankmates intercept everything, feed those fish at another spot before offering the filefish its portion. Several small passes can give it time to eat without leaving excess food.

Confirm swallowing during acclimation. A filefish may inspect or pick at a piece several times before eating. If it keeps spitting food out, try smaller particles or a quieter feeding location. Fine frozen foods or enriched live prey can help a reluctant fish begin feeding, and balanced pellets can be added once accepted.

Track the fish’s body condition over several meals. Some individuals feed openly, while others take food close to shelter. Adjust portions to the amount actually swallowed and remove scraps that collect beneath ledges.

Temperament and Tankmates

The Whitespotted Pygmy Filefish is generally peaceful with unrelated tankmates. Choose small, calm fish that leave it room to hover and time to feed. Large predators, persistent fin nippers, and fish that monopolize every feeding location can make a small community display unsuitable.

Compatible individuals can be kept together in a larger aquarium with separate retreats and more than one feeding area. Observe each fish rather than assuming all members of a group are eating. A single specimen is a practical choice when space or access to food is limited.

Watch for repeated chasing, damaged fins, or a fish that stays hidden through several meals. Shift food to a quieter position, provide an alternate shelter, and separate incompatible fish if feeding or use of cover does not improve.

Reef Compatibility

The Whitespotted Pygmy Filefish is Reef Safe Caution. Captive-bred groups have been kept with hard and soft corals without observed nipping. Individual behavior can vary, and the fish’s natural diet includes small invertebrates. Monitor sensitive polyps and very small ornamental shrimp after introduction.

Offer a complete diet and watch the animals nearest its regular feeding and resting areas. A single investigative peck is different from repeated feeding at the same colony. If picking persists, separate the fish or the affected livestock. A peaceful fish-only display with live rock is also suitable.

Water Parameters

Temperature: 75–78°F (24–26°C) for aquarium-acclimated fish
Specific Gravity: 1.024–1.026
pH: 8.1–8.4
Alkalinity: 8–12 dKH

Use a fully cycled, well-oxygenated marine aquarium. Keep ammonia and nitrite undetectable, replace evaporated water with fresh water, and match replacement saltwater to the aquarium during water changes. Check circulation around sheltered areas after equipment maintenance.

Review nutrient levels as feeding increases and clean mechanical filtration before trapped food decomposes. Observe the fish’s usual resting position, color changes, appetite, and breathing so you can recognize changes in condition.

Common Considerations

• Adult Housing: Provide at least 20 gallons for one adult up to about 3 inches (7.5 cm). Use more room for a group.
• Temperature: Use 75–78°F for aquarium-acclimated fish and confirm source holding conditions for wild or newly imported individuals.
• Feeding Access: Offer fine foods near cover and confirm swallowing before increasing portions.
• Color Variation: Green or darker color and changes in visible spotting are typical for this species.
• Reef Caution: Observe sensitive corals and very small shrimp for repeated picking.
• Tankmates: Choose peaceful fish and separate feeding areas when competition develops.