There are several different types of reactors available for marine and reef aquarium systems.

In general, reactors serve one of two purposes:

  1. To replenish depleted elements in our aquarium water
  2. To remove dissolved organics or chemicals from aquarium water

In this short article, we'll explain what the most popular types of reactors are, what they are used for and which type of filter media you should use inside of them.

Media Reactors/Phosphate Reactors

The most popular reactors we carry are media reactors, also referred to as fluidized bed filters (FBF) or phosphate reactors. Media reactors have quickly become an essential part of many hobbyists' aquarium filtration systems. They are easy to setup, easy to maintain and affordable, even for reefers on a strict budget.

AquaMaxx Standard Fluidized Media Reactor

Media reactors typically employ an up-flow (also called reverse flow) design that pushes water from the bottom of the reactor up through the media and back out into your aquarium. The upward water flow tumbles the media around inside the reactor, allowing for greater contact between your tank water and the bacterial film on the media. It also reduces the likelihood of channeling.

The most common types of filter media used inside a media reactor are:

Media reactors take up very little space compared to wet/dry filter systems but can be just as effective at breaking down waste inside your aquarium. A great example of a biological media reactor is the Lifegard Aquatics Fluidized Bed Filter.

For more information about media reactors, read How to Setup a Media Reactor.

Calcium Reactors

AquaC RX-1 Calcium Reactor with media

In a reef tank, calcareous organisms like corals and clams along with calcareous algae, such as coralline algae, utilize the calcium dissolved in the water as one of the components to build their skeletons or shells. To keep a constant level in our reef tanks we need to supply calcium into the water column. One of the ways to accomplish this is the use of a calcium reactor.

A calcium reactor, in conjunction with a CO2 system, dissolves calcium-based filter media inside the reactor to replenish calcium and other minor elements in your aquarium water. For tanks with a high calcium demand, a calcium reactor is an invaluable tool that eliminates the need to dose 2-part calcium/alkalinity supplements.

Some hobbyists have the misconception that setting up a calcium reactor is difficult. In reality, nothing could be further from the truth. Sure, they might take a little time to setup. But you're a reef aquarium hobbyist—patience is clearly a virtue you already possess!

Read How to Setup a Calcium Reactor, watch our videos then set aside a few hours to get your own reactor up and running. If you have questions, contact us! We're here to help and tech support is always free.

The most popular brands of calcium reactor media we carry also happen to be among our personal favorites: CaribSea A.R.M. AragoniteTwo Little Fishies ReBorn Coarse Aragonite and D-D ROWAlith C+.

Kalkwasser (Kalk) Reactors

Precision Marine Kalkreactor KR620

Similar to a calcium reactor, a kalkwasser reactor's primary purpose is to replenish calcium that has been depleted by calcium-based organisms inside your aquarium. Many hobbyists regard kalk reactors as the best way to maintain calcium levels inside a heavily stocked tank with Acropora, Montipora and other stony corals and clams. They can be used as a standalone or in addition to a calcium reactor.

The word kalkwasser (shortened to kalk most of the time) is a German word for “limewater.” When calcium hydroxide is mixed with RO or RO/DI water, it creates a solution that is rich in calcium, carbonate and also has a very high pH, normally above 12. This mixture can be very beneficial to a reef aquarium: it will not only supply calcium and carbonate, but it can also help bump up the pH, a problem many hobbyists contend with.

Our staff doesn't favor one kalkwasser mix over another (we like them all), but our customer's top-rated picks are from E.S.V., Seachem and Kent Marine.

Zeo Reactors

AquaMaxx ZEO Reactor Starter Kit

Zeo reactors utilize a unique zeolitic filter media to cultivate beneficial bacteria on zeolite rocks. While dissolved organics are removed through adsorption to improve water quality, beneficial bacteria are released as a food source for coral and other suspension-feeding organisms. Zeo reactors provide a colonization site for the microorganisms responsible for nitrification, denitrification and organic waste degradation in marine aquaria.

Zeo reactors help establish biological filtration to maintain an ultra-low nutrient environment, perfect for small polyp stony (SPS) corals like Acropora and Montipora.

If you are interested in learning more about Zeo Reactors, check out our recently established Probiotics and Labile Organic Carbon Dosing forum moderated by aquarium expert Murray Camp. Murray's forum discusses carbon dosing and probiotic methodologies, including DIY (vodka, etc.) and commercial (ZEOvitProdibio, BioPellets, etc.) systems, including dosing, troubleshooting and the science behind these husbandry methods.

Nitrate Reactors

Deltec NFP Nitrate Filter

Sometimes despite our best efforts to reduce the nitrate levels in our aquarium, nothing gets the job done. This is where a nitrate reactor comes in.

You're probably asking yourself, "How does a nitrate reactor work?" It's quite simple, really.

Aquarium water with a high nitrate level flows through the reactor media where bacteria is formed to break down the nitrate. The chamber inside the reactor becomes low in oxygen, so the bacteria will use the oxygen in the nitrate (NO3), thus breaking it down. It takes some time for these filters to become biologically active, so it may be 1-4 weeks before you see signs of lower nitrate levels.

All you'll need to get started is the reactor itself, media (if not included) and a feed pump (if needed or not included). Most nitrate reactors are designed for large aquariums where high fish populations are the norm. While they are occasionally seen on smaller tanks, they are usually best on aquariums of 55 gallons or more.

Korallin offers a great media for most Nitrate reactors.


Aquarium reactors help maintain water quality and replenish needed elements. When set up and run properly, they can make our systems look their best and keep livestock healthy. If you are having water quality issues or would like an easier way to dose the elements being utilized by your aquarium's inhabitants, you may want to consider adding a reactor to your system.