In reef aquariums that have a large water volume or high calcium demand maintaining calcium and alkalinity can be difficult using additives. Using a calcium reactor can make this process much easier.

Calcium reactor designs vary considerably in complexity but ultimately all follow the same principles, a vessel is filled with a calcium based material and water is circulated through it. In order to facilitate the dissolution of this calcium material the water being circulated through the vessel has carbon dioxide introduced to it in order to lower its pH. Most modern designs use a pump that circulates the water in the reaction chamber independent of the output to further maximize dissolution of the media.

Here’s a picture of a simple reactor design.

Water is fed through the orange line on the left side of the reactor, the saturated water will exit the reactor through the light green line labeled effluent at the same rate. The blue pump and plumbing are responsible for recirculation of the water through the reactor and introducing the CO2 into the system via a venturi on the output side of the pump.

Putting it All Together

The first thing you will need to put together your calcium reactor is a CO2 bottle and regulator. The CO2 bottle is a basic unit easily acquired from a welding supply shop or gas supply company. You will need a dual gauge regulator with a solenoid, needle valve and bubble counter like this Milwaukee Model.

The right valve on the regulator shows how much CO2 is in the bottle, the left one the pressure in the line to the reactor. Some people choose to forgo the solenoid but this is not advisable, in the case of a power outage the solenoid will close and cut off CO2 to the reactor. Without water flow in the reactor a CO2 build up can liquefy all of the media causing quite a mess. You can also use a pH controller to control the reactor better via this solenoid. The needle valve is used to make fine adjustments to the CO2 rate, filling the bubble counter with liquid and observing the rate of the bubbles going through it will allow you to keep track of this.

This is a fairly simple reactor fed with an aqualifter pump and using an eheim circulation pump. First rinse the media out and fill the reactor. Now connect the feed pump and ensure that the water is flowing through the reactor properly. Your reactor should have a sponge of some kind that keeps any small particles from getting stuck in the effluent hose, make sure this is in place. Direct your effluent hose into a high flow location in your system, preferably in the sump. For the initial setup or if you are going to be using a pH controller with a reactor that doesn’t have a pH probe holder you will need to direct the effluent into a cup so you can capture and test the effluent.

Please note, the flow rates in this guide are a basic starting point that should work with most reactors. Your reactor should come with instructions that give rates for that model, if they are different use them instead.

After you have as much of the air out of the system as possible start up the circulation pump and ensure it is working. Use silicone air tubing to connect the CO2 tank to the reactor and open the tank valve. Using the valve on the front of the regulator open the valve until the valve indicates you have 12lbs of pressure. Now open the needle valve until you have approximately 10 bubbles per minute coming out of the reactor. You should have a valve of some kind on the effluent line, adjust this until you get 40-60 ml/min out of it.

It will take some time for the pH of the water in the reactor to start dropping depending on the volume of your reactor. Your target pH is going to depend on the type of media you are using, aragonite media like Carib Sea ARM will dissolve at a pH of 6.5 but a calcite material like Schuran will need a pH of 6.2. If you have a controller, set the pH set point to your target and plug the solenoid into it. If you are adjusting your reactor manually adjust the CO2 rate via the needle valve until the pH of the effluent is stable. It will take hours for any adjustments to take effect so go slowly and double check often.

How Much Do You Need?
Matching the output of you reactor to the demand in your tank will take a while and some experimentation. The effluent will have a balanced amount of calcium and alkalinity in it and generally alkalinity will be easiest to test for, for each 1 meq/l of alkalinity provided in the effluent you will get 20 ppm calcium. Monitor the alkalinity of your tank for the next few days. If the addition is raising your alkalinity too much you can reduce the output by either increasing the effluent rate or lowering the CO2 rate. Conversely raising the CO2 rate or lowering the effluent flow rate will increase the amount of alkalinity added to your tank.

Alkalinity is Low but Calcium is High
This situation or the opposite can still happen when you are using a calcium reactor. Increasing the output of the reactor can’t fix this problem because it adds calcium and alkalinity in a balanced way. You will need to make an adjustment to the parameter that is low using an additive to bring things back in balance. It is also a good idea to check your magnesium level, as low magnesium can make it difficult to maintain balanced calcium and alkalinity.

You may need to regularly supplement magnesium to maintain a proper level. You can do this with your rector by adding a small amount of pure dolomite to the media or with a magnesium additive.

Tank pH is Falling

Excess CO2 dissolved in the effluent can bring the pH of the tank down over time. You can confirm the CO2 is the cause of this by putting some tank water in a cup and running the output of a small air pump into it for 6 hours, if the pH goes up it is because CO2 has outgassed. Some people only recommend running the reactor when the main lights of the system are on and the livestock is using CO2 for photosynthesis because of problems like this. Plug your solenoid into your light timer to if you are able to meet the demands of your system with the reactor only running half time. If you have a refugium try cycling the light so it is on when the main tank lights are off, the macro algae in the refugium will remove some of the excess CO2 for you while the display is dark.

If you still have pH problems you will need to find a way to remove the CO2 from the effluent. Direct the effluent into a media reactor filled with reactor media. You can also direct the effluent into a cup that then drains into the sump and add an airstone to outgas the excess CO2. Raising the pH of the effluent in this manner can cause some of the carbonate material in solution to precipitate so it will require regular maintenance.

Ongoing Maintenance

Often as soon as you get your reactor dialed in to keep pace with your system’s demand it will change. Regularly testing recording your calcium and alkalinity levels will allow you to perform any adjustments required, make them slowly.

* Regularly check the effluent pH and flow rate to make sure it is staying where you need it to be. The circulation pump should be cleaned once a month, remove any carbonate build up with vinegar.
* Check all CO2 hoses and connections for leaks. You don’t want dangerous levels of CO2 building up in your house.
* Check the amount and consistency of the media regularly. The replacement time for the media will depend on the rate of dissolution.