At first glance, a calcium reactor can be a bit intimidating with a seemingly complex network of tubes and conflicting advice all over the internet.


Explained the easy way, a calcium reactor is simply a device that is dosing calcium and alkalinity to your reef tank via a single concentrated solution. Much of the frustration and mystery around calcium reactors stems from old school methods and the lack of a simple, straightforward approach that works for any reefer.


Vertex calcium reactor on synergy reef sump


We put together this easy to understand guide to explain how a calcium reactor works for what we feel is the highest percentage path to success. Once you understand these basics, it is easier to develop your own personal approach to suit your tank’s needs, available space and personal budget.


To explain how a calcium reactor works, we are breaking it down into three distinct sections to easily establish an approach that anyone with the will and wherewithal can follow. The why, what and how of calcium reactors.


SPS coral in a reef tank


Why reefers use calcium reactors

We started our mission to explain calcium reactors to the masses with an entire video about the advantages of a calcium reactor over traditional methods of supplementation such as two part calcium and alkalinity supplements.


Check out the video! >> How much is a calcium reactor vs two part dosing. Is it worth putting on a reef tank? Reef FAQ


•More economical for large or high demand reef tanks.

•Does not increase salinity like some two parts do = less maintenance.

•Includes some critical trace elements.

•Cool factor of being environmentally conscious. Recycle old coral to grow new coral.

•The ultimate experience for gear junkies.


We found these advantages to be the most legitimate reasons someone would choose to use a calcium reactor and, of course, there may be even more advantages that apply to you and your tank.


Acropora Echinata Skeleton


If you are keeping stony corals, LPS and SPS, you likely already understand the fact that these corals uptake calcium and alkalinity from the reef aquarium water to form their calcium carbonate skeletal structures.


As they deplete the available calcium and alkalinity in the water, you need to replace it. 99% of us will start this major element replenishment with the use of two part calcium and alkalinity solutions.


BRS Pharma Two Part Solution


The reason most hobbyists choose to use 2-part is because it is easy to use. Balanced two part solutions supply a stable concentration in each of the two solutions. Dialing in how much you need to dose is as simple as testing the aquarium water and dosing a little more or a little less based on your tanks demand.


A calcium reactor is an alternative method of calcium and alkalinity supplementation and does the exact same thing as 2 part solutions.


The only difference is it includes both calcium, alkalinity and some trace elements in a single on-demand solution often referred to as the “effluent solution” as it exists the calcium reactor.


After the equipment investment, it is a cheaper solution to supplementing the major elements in your reef tank and it is easier to do. The media is simply bits of calcium carbonate or old coral skeleton and when set up the easy way with a stable concentration, it is as simple as adjusting how much goes into your tank to meet your demand.


Reef Octopus CR100 Calcium Reactor


How a calcium reactor works - the concept

A calcium reactor creates the effluent solution by lowering the pH of the water inside the reactor with dissolved CO2 until the calcium carbonate media or old coral pieces dissolve and enrich the water with calcium and alkalinity.

For example, alkalinity in your display tank might be 9 dKH and the alkalinity of the water inside the reactor is far more concentrated and will be around 30 dKH. Slowly dosing the concentrated water from inside the reactor (effluent solution) will effectively raise the tanks dKH to meet your daily consumption.


Calcium Reactor Media close up


Calcium Reactor = old coral skeleton, melted into a concentrated liquid that is slowly dosed into your reef tank.


This is where things get a little murky because there are various approaches to how you set-up and control a calcium reactor. The harder old school methods require much more manual tuning and regular tinkering of both the CO2 and dose rate to ensure you are getting the right amount of calcium and alkalinity. That is the source of all the horror stories and frustrations.


Fortunately, thanks to modern equipment, there is a far easier approach that results in a consistently stable solution at all times so all you need to do is adjust how much goes into your tank.


Advanced calcium reactor setup


What is required to make calcium reactors easy

There are essentially two steps to making a calcium reactor easy. Set the reactor up to make a stable concentration then set the right dose. Effectively the same mentality as two part solutions but you have a reactor that produces one part calcium and alkalinity solution, on demand.


Milwaukee pH Controller


STEP ONE - Creating a consistently stable concentration is the ticket because so long as you know the solution always has the same amount of calcium and alkalinity, dosing the right amount is just a matter of a little more or a little less effluent solution.


The easiest way to get a stable concentration of calcium and alkalinity from your reactor is use a pH controller to automatically control the amount of CO2 going into your calcium reactor. The pH controller will ensure the exact amount of CO2 is being fed into the reactor at all times to maintain the set pH.


For example, by pegging the pH inside your reactor at 6.5 with a controller, you are effectively pegging the concentration of calcium and alkalinity at 35 dKH.


Calcium reactor effluent solution concentrations from BRStv Investigates


That concentration scales up and down with pH changes. We recommend pegging pH safely at 6.6 pH which results in a concentration of about 30 dKH.


We demonstrated this in the BRStv Investigates episode How does Calcium Reactor potency change at different pH points? ANSWER! - BRStv Investigates


Once you have that stable concentration, you have an on-demand calcium and alkalinity producing machine.


Needle Valve on calcium reactor


STEP TWO - All that is left is figuring out how much to dose and adjust the rate at which this concentrated solution exits your reactor which is super easy.


For example, if you start with an effluent drip rate of 20mL per minute and your tank’s calcium and alkalinity levels are dropping, simply increase that dosage amount to 25mL per minute. If the levels are increasing, simply decrease that dosage amount to 15mL per minute.


This method is so precise there are even calculators to help you figure out exactly what the effluent flow rate and concentration should be.


Kamoer FX-STP Dosing Pump


To measure and set the dosage rate, the easiest option is to use a continuous duty dosing pump like the Kamoer FX-STP that has a dial to adjust the flow rate through your reactor. The digital display shows you the precise water flow conveniently measured in mL per minute.


Measuring calcium reactor effluent solution


Some higher end reactors, like the Vertex RX-C 6D Calcium Reactor, include a flow meter and needle valve for adjusting the effluent dose rate. You can also use a needle valve then collect and measure how much solution you get in one minute with a graduated cylinder or similar measuring device.



Everything we do here at BRS is to help you be successful with your reef aquarium, ultimately your success is our success. If you are exploring the idea of a calcium reactor, check out this reactor playlist we put together and start with our easy calcium reactor setup video that specifically goes into detail about how to set up a calcium reactor.