Calcium reactors and the word “easy” in the same sentence is sort of an oxymoron because reactors get a bad rap for being difficult. The facts are, once you understand the concept and get the right gear, it really is not all that hard and operation is automated. Our mission here is to squash the hysteria around calcium reactors and give you step-by-step instructions for the easiest calcium reactor setup. 

Vertex Calcium Reactor Setup

What Is a Calcium Reactor (CaRx)

A calcium reactor is a pressurized reactor full of calcium carbonate media. The most common media is crushed aragonite from dead coral skeletons or crushed calcite rock. Inside the reactor, tank water is injected with CO2 to lower the pH, that low pH environment dissolves the media and enriches the water with calcium and carbonate.

The enriched tank water is then dripped back into the aquarium effectively maintaining both calcium and alkalinity levels. You can also add a small portion of magnesium-based media into the reactor which then maintains your magnesium levels too. Depending on the exact media you are using, there are other elements that get dissolved into the water including a variety of trace elements and phosphate. 

Why Use A Calcium Reactor?

Calcium Reactors are best used on medium to very high demand reef tanks; SPS dominant tanks and mature mixed reef tanks with a heavy population of stony corals. They are quite convenient in that once setup, the single solution automatically maintains both calcium and alkalinity levels which eliminates the need for daily dosing. The operation is automated for the most part and increasing or decreasing levels can be done by simply adjusting the dosing pump. 

We do admit, a Calcium Reactor can look intimidating just because of all the plumbing and equipment, but operationally they are quite simple. All you need to do is create a consistent solution of calcium and alkalinity using a pH controller and then adjust how much you dose each day based on your tank’s demand.

Learn More With BRStv: BRStv Calcium Reactor Videos

pH Controller setpoint 6.5

How To Setup A Calcium Reactor - The Easy Way

1. Assemble The Reactor & Equipment

Each calcium reactor is a little unique in the exact way they are plumbed and need to be connected to your aquarium. Here is a complete shopping list of everything you need to get a CaRx installed but refer to the manufactures instructions specifically for assembly and connection to your tank. 

Mandatory Items:

 Optional Items:

Once you have everything, you will need to fill your CO2 Cylinder with food-grade CO2 which is available at restaurant supply stores or gas supply type retailers. When installing the regulator, be sure your CO2 cylinder is closed and follow the proper steps for opening the cylinder as instructed by your particular regulator. 

Fill the reactor with media, a 9:1 ratio of calcium reactor media to magnesium media is recommended. The reactor body can be about 90-95% full of total media. Connect your dosing pump, pH controller, and CO2 lines to the reactor; the check valve goes in between the reactor and CO2 regulator to reduce any chances of water backing up into your regulator which will ruin it. 

Once everything is connected, review the operating instructions for your dosing pump, CaRx reactor, pH Controller, and CO2 regulator before proceeding. You want to have a good idea of how to operate everything before you proceed.

2. Fill Reactor With Tank Water

Your dosing pump is what recirculates water from your tank, through the reactor then back.  It will take some time to fill up the reactor and you can run the dosing pump at a decent speed at this point to speed things up. Flow adjustment will come later. Let it run for 24 hours to purge out any trapped air and saturate the media. The water won't have any effect on water chemistry at this point; you just want to get as much of the trapped air out of the reactor as possible. Some mild agitation to the reactor body can help dislodge bubbles and push them out; you can also use one of the ports on top of our reactor to purge trapped air.   

After 24 hours have passed, reduce the flow rate through your reactor so that you're getting roughly 1-2 drops per second dripping back into your tank. This will be around 3-5mL per minute using the adjustments on your dosing pump but the precise flow rate really isn't relevant yet. The point is to start with a very slow flow that will be adjusted only after injecting CO2. 

Calcium Reactor Bubble Counter

3. Establish a Stable pH Using a pH Controller

The effluent solution being dosed into your tank should always contain the same concentration of dissolved calcium and alkalinity. A steady low pH inside the reactor is the ticket and is what maintains a stable strength solution. This is where the pH controller comes into play and will regulate the injection of CO2. 

The pH probe is installed to monitor the water inside your reactor. Set the pH controller setpoint somewhere between 6.3 - 6.8.  The most common setpoint is 6.5 pH but can vary a little depending on the media your using; just defer to manufacturer instructions. Connect the solenoid on your CO2 regulator to the pH controller power plug and you can proceed to carefully open the CO2 cylinder.  Adjust the CO2 bubble count to roughly 1-2 bubbles per second using the regulator adjustments. 

The pH controller will open and close the electronic solenoid that is attached to the CO2 regulator and only allow the necessary CO2 to maintain the desired pH of 6.5, no more and no less. It will take some time for the pH to fall, just be sure your dosing pump is moving water slowly and monitor the CO2 coming out of the regulator. 

You should see consistent bubbles of CO2 in the bubble counter and the pH controller should be ON, allowing CO2 into the reactor. It usually takes about an hour for that pH to fall into range but sometimes more depending on the exact bubble rate/flow ratio. The idea is to GO SLOW, you really want that solenoid to be open a majority of the time which means a very slow and steady injection of CO2. If pH fails to fall into the proper range, you can ever so slightly increase the CO2 bubble rate. 

Pro Tip: A CaRx is a good excuse to pick up an aquarium controller like the Neptune Systems Apex. The full Apex has two BNC ports for probe connection so you can monitor both the pH inside the reactor and in your display tank with the use of two pH probes. The entry level Apex EL only has one BNC probe port which will suffice to control pH inside the reactor. You could then upgrade with an additional PM1 module to add the second pH probe if you decide you need it down the road.

  1. Milwaukee MC122 pH Controller
    Milwaukee Instruments
    Milwaukee MC122 pH Controller



Calcium Reactor Effluent

4. Adjust The Flow Rate to Meet Your Tank's Demand

Once the pH drops into range inside the reactor as indicated by your pH controller, you now have consistent enriched water coming out of the reactor. This enriched tank water is often referred to as the "effluent solution".

The next process is dialing in the flow rate to meet your tank's demand for calcium and alkalinity using the dosing pump and test kits.   

  1. Be sure your tank's calcium and alkalinity levels are balanced at the ideal range before dialing in your reactor.
  2. Test and record your levels on day #1.
  3. Let the reactor drip slowly for 24 hours.
  4. Test and record levels again on day #2.
  5. If levels drop, increase the flow.  If levels are stable, leave the flow alone.  If levels increase, decrease the flow.  
  6. Test and record again on day #3 to verify stability or continue adjusting until stable parameters are tested for at least 3-5 days in a row. 

For example, if you start at the rate of flow at 10 mL per minute and the tested levels of calcium and alkalinity are indicating a DROP after 24 hours, turn up the flow to 15mL per minute and test again the following day. Repeat the process in small increments until you are able to maintain your desired levels. Changes are made in very small increments and no sooner than once every 24 hours. 

Not much different than dialing in a two-part solution which most of us have used at one point. In fact, just like two-part, there are easy-to-use calculators like Jdieck’s Calcium Reactor Set Up. Just enter a few fields and it tells you the exact flow rate to use.  You can measure the concentration of your effluent solution by running an alkalinity test on the water as it is exiting the reactor. It should measure somewhere between 25 - 40 dKH. 

Things To Consider

  • The approach to calcium reactors has evolved over the years with access to modern pH controllers and adjustable dosing pumps. While the concept has remained the same, this modern equipment makes it much easier to automate and tune a calcium reactor.  
  • Calcium reactor solution does contain a higher level of dissolved CO2 which means it can lower the average pH inside your display aquarium. Dripping effluent solution directly into a refugium or algae reactor will help remove that dissolved CO2 and eliminate the pH suppressing. 
  • Another solution to combat low pH with a CaRx is to use both kalkwasser solution and a calcium reactor together. Kalkwasser is dripped at night when pH is at its lowest and the CaRx can be dripped during the day when pH rises. Of course, you need to adjust the amount of each solution to accommodate your tank's needs.  
  • Increasing the flow rate through your reactor will decrease contact time with the media. Contrary to what other information you might find, this decrease in contact time has a very minimal effect on the calcium carbonate concentration of your effluent solution as long as pH stays stable in the reactor. Just peg your pH at 6.5 and adjust the flow to meet your demands. Watch BRStv Investigates - Calcium Reactor Flow Rates
  • Most calcium reactors will include some kind of valve to help control the flow rate but using an adjustable continuous duty dosing pump is far more consistent. The valve is really only needed if you decide to feed the reactor with tank water using a non-adjustable pump which is often a more affordable approach.
  • While the continuous duty adjustable dosing pump is an investment, it really is a game-changer in terms of adjusting the flow and finding the sweet spot for your particular tank. Manual flow adjustment valves tend to clog up and require more frequent cleaning.