The 15-minute Calcium Reactor

by Chris Paris for Reef Aquarium Information Depot

My FishRoom friend Rod Andrews told me about a single-pass (non-recirculating) calcium reactor that he made from one of those water filter cannisters that you can get at Home Depot and similar places. The simplicity of Rod's design appealed to me. I had one of those filters lying around, so I decided to build one, even though I already had a perfectly adequate single-pass reactor that had been serving me well for a year or so. Building this water filter-based reactor takes less time than reading this document.

At this point you may be wondering why a single-pass reactor is a good idea.

The particular water filter I started from is an Instapure K-50, from Teledyne Water Pik. I bought it at a local home center about three years ago. You can use whatever you can find locally, though these "plans" may not apply perfectly if you use a slightly different water filter.

Anatomy of the filter

These water filters consist of a clear plastic cylinder with a threaded lid at the top. The lid contains two connections: one for water input and one for water output. My filter has 3/4" female pipe threads for the connections. I prefer to premix the aquarium water and the CO2 using a bubble counter designed for that purpose, so my calcium reactors need only two holes in them: a single input and a single output. I discuss my bubble counter at the end of this article.

Look at the underside of the filter's lid. You can see that the water input port just empties out into the top of the reactor. The output port is different. There is a tube in the center of the lid, oriented vertically when the filter is in the normal upright position. The output port on the lid is connected to the inside of this tube. The tube is only a few inches long, basically extending down to the bottom of the lid, but not into the filter's body. We shall attach a PVC pipe to this tube, so that the output of the reactor will come from the bottom of the reactor. This ensures that no CO2 gas will leave the reactor.

The filter also has a small plastic ring that sits at the bottom of the filter, and serves as the base for whatever filter module you would normally use in the filter. This ring is a removable piece. We shall use this plastic ring to support the base of the tube that carries output water from the bottom of the reactor up through the output port of the lid.

Making the connection to the lid tube

We need a fairly water tight connection between the tube we add and the little stub tube on the inside of the lid. Fortunately, that stub tube is tapered, so we can get a nice seal just by sliding an appropriate fitting on. I found that a 3/4" PVC coupling works. The insides of PVC fittings contain a shoulder, which the PVC tube seats against. The shoulder in a PVC coupling is midway through the coupling, and is basically a small section of reduced diameter. It is this shoulder that seals against the filter tube, not the main inside diameter of the coupling. Since the diameter at the shoulder is not part of the "PVC spec," different manufacturers are free to use different dimensions there. I found that one coupling worked, whereas a coupling from a different manufacturer was too loose. You will just have to experiment with different fittings and pipe, and find something that works for you and your filter. If you have access to a lathe, you can easily bore out a pipe or fitting to the required dimension, but if you can find off the shelf parts that fit, then you can build the whole reactor using only a hand saw to cut PVC pipe.

Making the connection to the base ring

The plastic base ring inside the filter has three flexible prongs on it. These prongs fit inside the filter module that you would normally use in the filter. The prongs also fit nicely in a 3/4" PVC coupling. Now, we have a 3/4" coupling at the bottom of the filter, and one at the top.


We must cut a length of PVC pipe to cover the span between the lower and upper couplings. It can be somewhat tricky to get the length right. You have to push the upper PVC fitting into the stub tube on the lid to a certain depth in order to get a seal. If you do not get a seal, then nothing forces the water to pass through the aragonite bed, and the reactor may be ineffective. So the PVC pipe cannot be too short. But if the pipe is too long, you will not be able to close the reactor (screw on the lid) tightly, and it will leak.

What to glue

I glued the length of PVC pipe to the upper coupling, but nothing else. In particular, the connection between the top PVC coupling and the stub tube that is part of the lid must remain a slip fit, so that you can attach the lid with the tube already in the reactor, imbedded in sand, and so that you can remove the lid without pulling the tube out of the sand.

Connections to the filter

As I said earlier, my filter has 3/4" female pipe threads for the input and output. Many people prefer to use standard "airline tubing" for their reactor lines. Airline tubing is 3/16" ID. I prefer to use 1/4" ID tubing, which you can get at local home centers, and elsewhere. I purchased some 3/4" male thread to 1/4" hose barb connectors from MSC. The item number 48712525 at MSC gets you a 10-pack for about $2.50. MSC has a $25 minumum order.

Filling the reactor with sand

With the reactor open and the loose parts before you, fit the lower coupling onto the prongs of the filter's plastic lower ring. Plug the PVC pipe into the other side of the lower coupling. I am assuming that the other end of the PVC pipe has the upper coupling glued onto it. Now place the PVC pipe into the filter body, with the filter's lower ring at the bottom. The PVC tube may be somewhat wobbly at this stage, because nothing supports it except at the bottom, and the lower plastic ring in the filter can move around a bit.

While holding the PVC tube roughly centered in the filter body, dump your aragonite sand into the reactor. Fill it right up. Now screw the lid on. The lid's stub tube should fit into the top coupling of your PVC tube, and form a seal.

Filling the reactor with water

You now have the problem that the reactor is filled with sand and air rather than sand and water. Because the reactor's output comes from the bottom of the reactor, the gas in the reactor will be trapped. How are you going to get that air out? Turn on the water flow to a high rate (a solid flow rather than the drip that you will use in normal operation), turn the reactor upside down, and the air will exit the reactor. After all the air is out, turn the reactor right side up.

A bubble counter

Bubble counter CO2 enters through the top connection. The white object is a check valve. The gas is released into the bubble counter at the bottom of the rigid airline tubing that appears in the center of the main tube.

Water from the aquarium is pumped into the bottom connection by a small powerhead located in the sump of the reef setup, or by a tee off of the main system pump or the skimmer pump.

Some of the CO2 dissolves in the water within the bubble counter, and some exits the bubble counter still in gas form. A combined CO2 and water product leaves the bubble counter through the side wall of the tube.

From there, it goes through the plastic needle valve, which is used to control the rate at which the aquarium water passes through the calcium reactor. Premixing the CO2 and aquarium water lowers the pH of the mixture to about 5.8, and ensures that calcium carbonate is highly soluble in the fluid that passes through the valve. This high solubility in the valve prevents the valve from clogging with calcium deposits.

After water comes out of the righthand side of the needle valve, it will be taken via flexible tubing to the input of the calcium reactor.

At this point your reactor is finished and operational. I do not intend this document to teach you how to use a calcium reactor. From here on out all of the standard reactor usage lore applies.

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Last modified: Thu Feb 03 19:51:35 Eastern Standard Time 2000