Dive Deeper: Which Aquarium Reactor Do You Need?
"Reactor" covers five genuinely different pieces of equipment that solve different problems. Buying the wrong type is the most common mistake in this category, so start with the job rather than the hardware.
Calcium Reactors
A calcium reactor dissolves aragonite media with CO2-acidified water and returns the effluent to your tank, replacing calcium, alkalinity and magnesium in the same ratio corals consume them. It is the most stable long-term solution for a heavy SPS load, and once dialled in it needs very little attention. Expect to also need a CO2 cylinder, regulator, pH controller and a way to meter effluent. Reef Octopus VarioS and Deltec Twin Tech units cover the range from mid-size reefs to very large systems.
Choose a calcium reactor if: your daily alkalinity demand has outgrown two-part dosing, or you are running 150+ gallons of coral-dominant reef.
Media and Carbon Reactors
A media reactor fluidises GFO, carbon, biopellets or resin so every granule contacts water rather than channelling. Passive media bags do a fraction of the work. Fluidised reactors are inexpensive, easy to plumb, and the single best upgrade for a tank fighting phosphate or yellowing water.
Choose a media reactor if: you are running GFO, carbon, or any granular chemical filtration media.
Biopellet Reactors
Biopellet reactors host bacteria on a solid carbon source, tumbling the pellets to keep the bacterial film active. The bacteria consume nitrate and phosphate and are then skimmed out. They require a properly sized protein skimmer downstream to work — without it, you are just adding bacteria to the water column.
Choose a biopellet reactor if: you want carbon dosing without the daily measuring, and you already run a strong skimmer.
Kalkwasser Reactors and Stirrers
A kalk stirrer keeps calcium hydroxide in saturated solution and doses it as top-off water. It simultaneously replaces evaporation, raises alkalinity and calcium, and precipitates phosphate. It is the cheapest way to maintain a light-to-moderate coral load, and it pairs naturally with an ATO. The Reef Octopus KS250 Nilson-style reactor is the reference design here.
Choose a kalk reactor if: your alkalinity demand is modest and you want top-off doing double duty.
Macroalgae and Algae Reactors
An algae reactor grows chaeto under dedicated light in a contained vessel, exporting nitrate and phosphate as harvested biomass. It does the job of a refugium in a fraction of the footprint. The Tunze MAR-3181 is the compact option in this collection.
Choose an algae reactor if: you want natural nutrient export and do not have room for a refugium.
Sizing and Plumbing Notes
- Feed pumps matter. A reactor starved of flow will channel; a reactor over-driven will pack or blow media into the sump. Follow the manufacturer's flow spec, and use a valve to fine-tune.
- Mount for maintenance. You will open a media reactor monthly. Leave room to pull the lid and hose without disassembling the plumbing.
- Never plumb a reactor directly to the display without a check. Effluent from a calcium reactor sitting at pH 6.5 going straight into a display is how alkalinity crashes happen.
- Rinse new media. GFO and carbon dust will cloud a tank and irritate corals.
Brands We Carry
Reef Octopus — VarioS CR140 and CR200 calcium reactors, SRO CR3000D dual-chamber, KS250 kalk reactor, BR3000SS biopellet reactor.
Deltec USA — Twin Tech CRTT calcium reactors, PF fluidised-bed calcium reactors, FR fluidised media reactors.
Tunze — MAR-3181 macroalgae reactor.
IceCap, Bashsea and Innovative Marine — media reactors and compact fluidised units.
Complete the System
Pair a reactor with aquarium water test kits to verify effluent, an aquarium controller for pH control on a calcium reactor, and reef tank dosing pumps for supplemental two-part.
