A reactor is a sealed chamber that passes tank water through a specific media or substance to achieve a precise chemical effect. In a reef tank, the term covers several very different pieces of equipment: calcium reactors, carbon reactors, phosphate reactors, and nitrate reactors, each solving a different problem. Which one you need depends entirely on what your tank is lacking or what's out of balance.

Most reef keepers start with a protein skimmer and proper lighting, then add reactors as the tank matures and water chemistry demands become more specific. A calcium reactor becomes important once you have enough stony coral that Kalkwasser additions can't keep up with calcium consumption. A phosphate reactor matters when nutrient levels creep up despite other filtration. Knowing what each reactor does prevents you from spending money on equipment you don't need yet.

Calcium Reactors: The Core of Hard Coral Husbandry

A calcium reactor is designed to maintain stable calcium, alkalinity, and magnesium levels for stony corals (SPS and LPS). These corals constantly extract calcium and carbonate from the water to build their skeletons, and replacing that demand without a reactor means daily dosing or frequent two-part additions.

How Calcium Reactors Work

The reactor chamber is filled with calcium carbonate media (usually crushed coral, aragonite, or commercial media like ARM Coarse or Reef Saver Rock from BRS). CO2 is injected into the chamber, lowering the internal pH to around 6.5-6.8. At that acidity level, the calcium carbonate media dissolves slowly, releasing calcium and carbonate ions into the water that passes through the chamber.

The effluent exits the reactor and enters the sump, where it raises system calcium and alkalinity. The trick is dialing in the flow rate and CO2 injection rate to match your corals' consumption without overshooting.

Reactor size is typically measured by the chamber volume and the system size it can support. A Two Little Fishies C-1 Calcium Reactor works well on systems up to 75 gallons with moderate coral loads. The Aqua Medic 1000 and BRS Dual Chamber Reactor handle larger, more demanding setups with SPS-dominated tanks up to 200 gallons. For very large systems (300+ gallons), the GEO 612 or Korallin C-1502 are the industry standards.

You'll also need a CO2 cylinder, a CO2 regulator (Milwaukee MA957 is a reliable entry-level option), and a bubble counter. Budget about $300-600 total for a complete small-system calcium reactor setup.

Carbon Reactors: Maintaining Water Clarity and Organic Control

A carbon reactor passes tank water through a chamber of activated carbon to remove dissolved organics, yellowing compounds, and some toxins. The advantage over a passive bag of carbon in a sump is that the reactor forces water contact with the media, increasing efficiency. Carbon in a passive mesh bag sees maybe 20% of the water flowing through the area. A reactor ensures nearly 100% contact.

Two Little Fishies PhosBan Reactor 150 is commonly used as a carbon reactor (despite the name). It works with either activated carbon (ROX 0.8 carbon from BRS is popular) or GFO (granular ferric oxide) for phosphate removal. The chamber holds enough media for a 100-200 gallon system, and the pump forces water through at a controlled rate.

Run carbon in a reactor rather than passively only if you have a specific problem: persistent yellow tint, elevated dissolved organics after a system crash, or heavy stocking. Most established reef tanks with a good skimmer don't need constant carbon.

Phosphate Reactors: Controlling Algae and Coral Growth

Elevated phosphate (above 0.05-0.1 ppm in a reef tank) suppresses coral growth and calcification and feeds nuisance algae. Phosphate reactors use granular ferric oxide (GFO) as the media. GFO adsorbs phosphate and silicates as water passes through the chamber.

The same Two Little Fishies PhosBan 150 or BRS Single Reactor works for GFO. Media brands like BRS GFO (both standard and high-capacity), Salifert PhosEx Ultra, and Two Little Fishies PhosNet are available. Two-Part high-capacity GFO is coarser and less likely to cloud the tank if flow rate gets too high.

Important: introduce GFO slowly in an established reef tank. Dropping phosphate from 0.3 ppm to near zero in 48 hours can cause tissue recession in acclimated corals. Ramp up GFO gradually over 2-3 weeks.

Nitrate Reactors: Managing Nutrient Levels

Nitrate reactors (also called sulfur denitrators) use bacteria to convert nitrate to nitrogen gas in an oxygen-deprived environment. Water flows slowly through a chamber containing sulfur pellets or other carbon sources that feed anaerobic bacteria. These bacteria consume nitrate for energy and release nitrogen gas.

Deltec NR 509 and BRS Sulphur Denitrator Reactor are commonly used examples. Nitrate reactors require careful dialing in: too fast a flow rate and the bacteria die off; too slow and hydrogen sulfide (which smells like rotten eggs and is toxic to corals) can build up.

Most reef keepers with heavy stocking address nitrate through water changes, improved protein skimming, and refugiums rather than nitrate reactors, which are more maintenance-intensive. Nitrate reactors make more sense in semi-closed systems or in situations where water change frequency is limited.

Do You Need a Reactor Right Now?

If your tank is under 6 months old and you're keeping soft corals or LPS, probably not. Soft corals and most LPS tolerate a range of calcium and alkalinity values, and two-part dosing (BRS Two Part System or similar) handles their needs without a reactor.

Reactors become worthwhile when: - You're keeping SPS corals that consume calcium fast enough that two-part dosing costs exceed the price of a reactor - Phosphate is persistently elevated despite good skimming and nutrient control - Your tank is mature (12+ months) and you're optimizing for growth rate and coloration

Check out our guide to best aquarium equipment for a broader look at what each piece of reef equipment does and when to add it. Our top aquarium equipment roundup also covers reactor brands with user reviews and pricing comparisons.

Frequently Asked Questions

Do I need a calcium reactor for soft corals? No. Soft corals like leathers, zoanthids, and mushrooms have very low calcium demands and tolerate the range maintained by regular water changes and two-part dosing. A calcium reactor is most useful when you have significant SPS coral coverage that demands consistent high calcium (400-450 ppm) and alkalinity (8-10 dKH).

Can I use one reactor body for both carbon and GFO? You can run separate reactor chambers in series. Dual reactor bodies like the BRS Dual Reactor let you run carbon in one chamber and GFO in the other with a single pump. Running them together in the same chamber isn't recommended because carbon and GFO media have different flow rate requirements.

How often do I change reactor media? Calcium reactor media (aragonite) dissolves slowly and typically needs replacement every 3-12 months depending on coral load and how fast the media is being consumed. GFO is typically replaced every 4-8 weeks when it becomes exhausted. Activated carbon should be replaced every 4-6 weeks or when water starts yellowing again.

What's the biggest mistake people make with calcium reactors? Running the effluent pH too low. If the internal chamber pH drops below 6.4, you're dissolving media too fast and the effluent carbonate levels can cause issues in the sump. Target a chamber pH of 6.5-6.7 and an effluent drip rate that keeps calcium and alkalinity stable without pushing them above target values.

Reactors are precision tools for specific reef chemistry problems. A calcium reactor solves the dosing grind for SPS-heavy systems. A phosphate reactor controls the nutrient issue driving algae and coral stress. A carbon reactor keeps water optically clear. You don't need all of them, but knowing what each one does helps you recognize when one has actually become necessary.