Every calcium reactor has a primary chamber that recirculates CO2 and melts the calcium reactor media, however, some reactors have a secondary reaction chamber intended to be filled with additional calcium carbonate media. This secondary or effluent chamber is then supposed to increase the pH of the solution by removing some of the dissolved CO2 and also increase the calcium carbonate concentration because of the longer contact time with the media.

Calcium Reactors with Secondary Chambers

We put this to the test in order to answer two questions:

  • 1. Will secondary calcium reactor chambers, intended to off-gas excess CO2, really result in a higher pH of the effluent/outlet solution and therefore help to maintain a higher pH in the display?
  • 2. Will the secondary chamber increase the potency (amount of dissolved calcium carbonate) of the calcium reactor’s effluent solution?

Neptune Systems pH Data Log Graph

Calcium reactors will typically cause a slight drop in average pH values inside your display, this is universally known. If we can prove that this secondary chamber will mitigate that drop in pH, reefers will find a benefit.

Along with that, a higher concentration of calcium carbonate in the effluent solution will mean less solution required for maintaining calcium and alkalinity levels resulting in an even further reduced impact on the pH inside your reef tank. A win/win for a majority of reefers! 

Vertex Calcium Reactor and Two Little Fishies ReBorn Media

The Experiment

We set up a Vertex RX-C 6D Calcium Reactor which integrates the secondary pH chamber into its design. We filled it with Two Little Fishies Reborn Calcium Reactor Media, a hobby favorite with a reputation for being clean and free of impurities. 

We tested two common pH set points of 6.4 and 6.8, which represent the lower and upper pH set points that most reefers will use when controlling a calcium reactor based on pH. We took pH readings at different flow rates of 20, 60 and 100 milliliters per minute (mL/min) to see how this varying contact time affects the pH in between the primary and secondary chamber.

Bulk Reef Supply Double Junction pH Probe

In order to identify the change in pH we need to record readings before and after the secondary chamber. We can take the pH set points of 6.4 and 6.8 as our reading for the primary chamber since this will always be the environment maintained inside the recirculating primary chamber. We used an inline BRS Dual Junction pH probe attached to an American Marine Pinpoint Ph Monitor to measure the effluent solution as it exits the secondary chamber. 

Hanna Instruments Alkalinity Checker Colorimeter

To test for changes in the calcium carbonate concentration, we are taking Alkalinity readings before and after the secondary chamber. We rigged up a three way ball valve before the second chamber which allowed us to draw a sample of the solution directly from the main recirculating chamber. Then we simply collected samples of the effluent solution coming from the secondary chamber and tested using hobby grade test kits.

Changes in pH

We first looked at any changes in the pH value from the set point of 6.4 after the secondary chamber. Measurements were recorded at each of the different flow rates of 20, 60, and 100 mL/min.

Change in pH Values with 6.4 Setpoint

At a pH of 6.4 and flow rate of 20 mL/min, we see a slight pH increase of 7-hundredths to 6.47pH after the secondary chamber. The same 0.07pH increase remained constant at the additional flow rates of 60 and 100 mL/min.

Change in pH Values with 6.8 Setpoint

At a pH of 6.8, we found again a slight increase at the 20 mL/min flow rate from 6.8 in the main recirculating chamber to 6.85 after the secondary chamber. With the flow cranked up to 60 mL/min, we gained another 100th of a pH point to 6.86 and at 100 mL/min we reached a pH of 6.87.

So this slight increase in pH may not seem like much to many of you. For those of you, however, running a calcium reactor and maintaining a pH in your display around 7.9, this slight bump with a secondary chamber may just be the ticket to push you up to 8.0.

Change in Alkalinity with 6.4 Setpoint

Changes in Alkalinity

Starting with the 6.4 pH set point at 20 mL/min, before the secondary chamber we saw an average alkalinity of 35.4 dKH inside the primary chamber of the calcium reactor. After it traveled through the small secondary chamber, it gained about 8.2 dKH for a total of 43.6 dKH. 

When we increase the flow rate to 60mLs per minute, which means less contact time, we still see an increase in saturation but it is less dramatic with a total increase of about 3 dKH. At 100 mL/min the results were as expected with an even less pronounced increase in saturation from 32.8 dKH before the secondary chamber and 34.6 after. 

We found a total average increase of 13% in dKH values when combining the results of all three flow rates at a pH set point of 6.4 inside the calcium reactor.

It seems that the longer contact time with a slower flow through rate certainly results in a more concentrated effluent solution because the water has more time to break down and dissolve more media.

Change in Alkalinity with 6.8 Setpoint

At the 6.8 pH set point with the same flow rates we see that at 20 mL/min we have a 21.6 dKH average before the secondary chamber followed by a slight increase to 23.2 dKH after, for a full change of 1.6d KH. 

At 60 mL/min flow, that change falls to 1.1dKH, and at 100 mL/min the change is again less dramatic from 17.8 dKH to 18.7dKH. 

The average change in concentration across the three flow rates at the 6.8 pH setpoint was about 6% which shows the increase in alkalinity was less pronounced at a higher pH set point.

Reef Fantasy Score

Will secondary calcium reactor chambers, intended to off-gas excess CO2, really result in a higher pH effluent?

We give this one a 4 on the reef fantasy scale. Although there was a slight increase in pH from the main chamber to the secondary chamber, it really wasn’t as impactful as we thought it would be. 

These small incremental changes in pH may be beneficial to a small group of reefers looking to gain an edge on pH in any way possible. For most hobbyists using a calcium reactor and struggling with low tank pH to begin with, a more substantial gain in pH would be desired.

Alkalinity Concentration Comparison

Is there any difference in max saturation between the main chamber and the secondary chamber?

We found a significant increase in alkalinity after the secondary chamber when running the lower pH setpoint and slower flow rates. As you increase both the pH and flow through rate, the impact on total saturation becomes less dramatic.


If your display is on the verge of being too low, or if you’re at the upper end of a pH point and are looking to gain a little edge, you could add on a secondary chamber for a very small increase in pH, no more than a few hundredths of a pH point.

Popular methods to increase pH values in a reef tank

For those of you looking for more significant changes to pH, you'll likely be better served by looking at other ways to increase your tank’s overall pH. Better ventilation or opening a window in the room, supplementing your ATO with Kalkwasser, adding a refugium, or even attaching a CO2 scrubber to your protein skimmer will all have a more drastic impact.

For those reefers looking to optimize the saturation levels of calcium reactor effluent solution, adding a secondary chamber will achieve this. Most notably when running the reactor at a lower pH and slower flow rate.

SPS dominate reef tank with a high demand for calcium and alkalinity

This is very ideal for those of you with a considerable demand for calcium and alkalinity because you can meet your tanks demand with less solution per minute. This will result in an even further reduced impact on the display tanks pH levels compared to running the same reactor set-point without a secondary chamber.

With such eye opening results and a successful test model, we will definitely explore the saturation levels of a few other popular calcium reactor media types such as CaribSea ARM Coarse and Brightwell Aquatics CoralAzarus in future BRStv episodes.

Related Content _______________________________________________________

Watch Video: How does Calcium Reactor potency change at different pH points? ANSWER! - BRStv Investigates

Watch Video: Does the calcium reactor flow rate REALLY matter? | BRStv Investigates

Watch Video: How to change from 2 part dosing to a Calcium Reactor? - BRStv ReefFAQs

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