calcium reactor additional chamber
Calcium Reactor Modification

by Justin Hester

Recently we did some modifications to our calcium reactor that has resulted in a leveling of our pH like we have never seen before. Its also made our coral growth go bonkers!  We have literally had some of our acro’s put on monthly growth in about a week after making this one tweak to our setup!


What is going on in your calcium reactor

We have run a GEO 818 calcium reactor on our main setup since about 2009 and to be honest, adding it was a game changer for us as far as growing corals.  Shortly after adding it though, we had a problem with the pH in our systems being a little low and daily swings of .2-.3 pH that made us cringe.

Calcium reactors work by taking a chamber filled with aragonite or dead coral skeletons and making that water acidic via CO2 in order to dissolve the aragonite.  This solution is then metered into the reef aquarium at a rate that keeps your alkalinity as steady as possible between 7 and 9dkH ( 140-155ppm).  By doing this, you are also dosing back all other basic elements that corals should need to grow including the big ones like calcium and magnesium, and lesser focused on elements like molybdenum and trace elements.  Sounds like the reef nirvana doesn’t it?  Well there is a major hitch that we and many other reef aquarists have hit with this setup.  The effluent you drip back into your tank has a pH that is super low compared to what you are trying to keep your tank at.  Something like 6.5-6.7pH compared to suggested reef tank pH of 8.3.

The other problem with dosing this low pH solution into your tank is that reef tanks have a natural dailly pH fluctuation based on photosynthesis.  This produces more O2 during the day which adds a slight raising of the pH and then during respiration at night, your pH will decrease as CO2 is produced.  The addition of your low pH calcium reactor effluent basically dampens the pH rise during the day and enhances the drop at night, leading to the nasty swing that corals do not particularly like.

Knowing this wasn’t going to bode well for our corals’ health, we read online that many hobbyists with calcium reactors also added Kalk reactors which will basically feed top off water through a solution of calcium hydroxide before it drips slowly back into your sump.  The downside to this method is that Kalk is super high in pH like 12.8, so you really need a controller on your setup in order to keep from dosing too much.  Shortly after adding out GEO 818, we added their Kalk 616 and modified it a bit to fit our top off setup and keep our pH from dipping below 8.

The picture you start to get here is that by doing something you thought was good, you are now playing a precarious game of ping pong with your tank pH and hoping nothing causes the ball to fall off the table!

Calcium Reactor Modification

Ok, so back in July, I was staring at my sump like we all do from time to time.  Pondering what I could do in terms of modifications to leverage the benefits of the calcium reactor, but mitigate the pH swings we were seeing from 7.9 or lower at night up to 8.25 during the day.  I had seen many a reactor fitted with a second chamber in order to “burn off” the excess CO2 through dissolution of more aragonite.  Think of this as adding a side-car to a motorcycle in order to steady it out.  I decided to take an old jumbo BRS media reactor I had on the shelf and try to fill what was the bio-pellet chamber with aragonite and then aim my effluent from the reactor through this final chamber before the water returned to the tank. After setting this up, I decided to measure the pH of the straight effluent from the main reactor and then the pH from my “new” little side chamber.  The results were as follows:

Inside calcium reactor: pH 6.53

Calcium reactor effluent before NEW chamber: pH 6.58

Calcium Reactor effluent after NEW chamber: 6.77

Thinking this was not a bad fix, I left it for a few weeks and definitely noticed that our daily pH swings had overnight been reduced to about .15 which was much better than .3 or more.  Last week, I was down puttering around with the tanks and kept staring at the new chamber and started to wonder if there was even a step farther I could take this modification to make our daily pH swings be even closer to what occurs in the ocean.  An idea came to me from how nature does it!

APEX PH Variation

Daily range before adding chamber and then right after we added it


After adding the drip chamber













During the day I thought, O2 from photosynthesis is what gives a slight raise to pH, so how could we introduce more O2 into the water that was coming out of the CO2 rich calcium reactor environment.  What I came up with was to take the aragonite cylinder out of the jumbo reactor and basically prop it up and drip my calcium reactor effluent through it.  The water would trickle from rock to rock like a pinball and gain O2 each time it splashed and interacted with the air.  It seemed like something that was easy to try and easy to test its results after. So I removed the cylinder added the effluent tube into the top and then after a few minutes tested the pH after the new chamber.  To my surprise, it was now coming out at 7.2!

After a week of having this setup I am happy to report that our tank pH has literally been 8.30/8.31 for almost the entire week straight!  I still have the kalk reactor hooked up, but I may try pulling that and seeing just how steady the calcium reactor modification can keep the pH.

Conclusions and next steps

Given the success of this modification so far, I have a couple things I want to try.  First, since my setup currently is a bit of an eyesore, I want to pretty up the solution so that its a more integral part of our reef equipment.  I am going to try and take a 3-4ft piece of PVC and fill it with aragonite in hopes it will increase the effluent’s O2 contact time and further raise the pH before it returns to the aquarium.  I’m thinking if I can reach 7.5 or more, the Kalk reactor will not be needed to counter the pH of the tank, but we will see.

Any thoughts or ideas for further modification of a calcium reactor?  We would love your feedback.


For a full write up on what a calcium reactor is and what is does in your reef aquarium, see our write up here

Justin Hester ReefNationJustin has spent most of his life around the ocean, growing up in Long Island by the water and most recently with his work at ReefNation.  Traveling the world and seeing first hand many of the man made problems our oceans face today, Justin has used ReefNation to contribute a thousands of captively grown corals from almost 200 species to reef hobbyists and stores around the US.