Introduction
As every serious marine aquarist knows, water chemistry is a huge deal in reef aquariums, and there are many methods to help maintain good water quality. There are so many options that it can be overwhelming on first glance. This article looks at one of these technologies, the calcium reactor.

Right off the bat, it has a scary name. Calcium reactor. Despite its intimidating name, a calcium reactor is actually not THAT complicated. It is essentially a chamber that recirculates water and slowly dissolves calcium carbonate media by slow injection of carbon dioxide.

How does it work?
Carbon dioxide when mixed into water converts to carbonic acid, thus lowering the pH of the water in the reactor. When the water inside the reactor becomes acidic enough, the media starts to break down and release calcium and alkalinity into the water, making it bioavailable to the tank inhabitants. By slowly reintroducing this water back into the aquarium, a calcium reactor helps maintain calcium and alkalinity levels. These two are major water chemistry parameters for stony corals and other tank inhabitants such as clams. The more stony corals in the reef, the higher their demand for calcium and alkalinity to continue growth, especially fast-growing stony corals such as Acropora.

Unfortunately, this process is also happening in the ocean as carbon dioxide levels rise. If you have ever heard the term “ocean acidification,” that’s what is being described. The ocean itself is becoming more acidic and is preventing stony corals from growing–or worse, dissolving their skeletons while the coral is alive. It is part of the reason many of the reefs around the world are struggling or bleaching out altogether.

Why a calcium reactor?
The reason I like using calcium reactors so much here at Tidal Gardens is they are fairly low maintenance once set up and do a good job of maintaining calcium and alkalinity. Low maintenance is important here because there are so many other things in the systems that require attention. Also, if there is a problem with the reactor as a result of neglect, the reactor just stops working (it doesn’t dump a year’s supply of additives into your tank like a dosing pump that goes rogue).

The various parts of the reactor
At first glance, a calcium reactor appears complex, but it is a relatively simple device. Let’s take a quick look at all the parts in turn.

The Reaction Chamber
The chamber is usually a large acrylic tube with as access port at the top to easily fill and refill calcium carbonate media. The top has to be secured down to avoid leaks once the reactor is filled with water and starts recirculating. The one pictured accomplishes this with a large orange O-ring and thumb screws around the entire circumference of the top plate.

The Media
The most commonly used media is aragonite, a great source for calcium carbonate. There is some debate about the ideal size of the media. Smaller media has more surface area; however it is more likely to clog as it begins to dissolve into sludge. Larger media has less surface area but retains spaces for water and CO2 to flow through long into operation, making it less likely to clog. I personally like to use media that is on the coarse side, because it allows for less-frequent maintenance and the loss of surface area can be made up using a larger reactor.

Two Pumps Required for Operation
A calcium reactor requires two pumps: a feed pump and a recirculating pump. A feed pump sends water to the reactor and maintains positive pressure in the unit. It is important to get one strong enough to force water into the reactor while under a little bit of back pressure because as the media dissolves, it turns a little slushy, and also the output of the reactor is going to be stopped down to a controlled drip.

The pump that does all the heavy lifting is the recirculating pump. This particular reactor takes water from the top of the unit and sends it to the bottom of the reactor. The CO2 gets chopped up and diffuses back through the media in a constant loop.

The water leaves the reactor and heads back into the system through the effluent line. It is a little counter-intuitive to send low pH water back into the system, but this actually helps boost the pH in the aquarium and add buffering stability. When the reactors here are running well, our pH is a rock-solid 8.3, and the tanks have steady levels of both calcium and alkalinity.

Carbon Dioxide System
The CO2 portion of the system requires a tank and regulator, as well as a needle valve to control the flow of gas to the calcium reactor. It is a hidden cost to the reactor. Often aquarists price out a calcium reactor but fail to account for the CO2 portion. It’s usually a few hundred dollars for the tank and regulator, depending on the size of the CO2 tank and the quality of the CO2 regulator and needle valve. The regulators I’ve seen for the saltwater hobby are fairly inexpensive compared to the very high-quality ones used by the freshwater planted tank folks. I’ve seen some CO2 setups for the freshwater folks into the 4-figures.

In any case, there is a connector for the CO2 on the reactor that goes to a bubble counter. The bubble counter is a quick and easy visual reference for how much CO2 is going into the reactor. I shoot for about 1 bubble per second or less. The real way to determine whether the reactor is working is to test the pH. A well-designed reactor has a port on the lid for a pH probe, so that the acidity of the water can be constantly monitored. If your reactor model does not have a pH probe port, you can have the effluent first pour into a cup, and a measurement can be taken there before going into the tank.

I shoot for a pH a little above 7. In my experience, when the pH is lower than 7, the reactor is a milkshake, and if it is up there around 8, there is probably no CO2 flow at all; either the needle valve needs to be tweaked or the gas tank is empty. Our 10-lb. CO2 tanks last about 3-4 months, and when they run out, it is a good time to take the reactor apart and clean everything and top off the media.

Conclusion
So, in summary, water chemistry is a really big topic, and there are a lot of different ways to achieve good chemical balance for your corals. Hopefully this article was helpful in helping you understanding one of these technologies. The calcium reactor is a great choice for a low-maintenance device that gently stabilizes calcium and alkalinity.