At first glance, a calcium reactor appears to be a complicated piece of equipment. Novice aquarists are often intimidated by all the special equipment involved, and it doesn’t help that most of the reactors available to hobbyists are downright difficult to use.

It’s a shame, because a well-designed calcium reactor is one of the most valuable pieces of equipment available to the reef aquarist. It can save a significant amount of time, hassle, and money. The stabilizing effects of a calcium reactor leads to a healthier, more successful reef tank.

Although the initial cost of a reactor is expensive, most aquarists end up saving money in the long run. The price of adding dry or liquid supplements adds up fast.

As the organisms in your aquarium grow, many remove calcium and alkalinity from the water column. Corals, coralline and calcareous algae, mollusks (snails, etc.), clams, and numerous other invertebrates all need calcium and alkalinity to survive and reproduce.

A calcium reactor maintains the level of calcium and alkalinity within a reef aquarium. More importantly, a reactor maintains the proper balance of calcium to alkalinity, which is critical over the longterm.  Where calcification is concerned, these two values are intimately related (one affects the other, and vice versa).  Most other methods of supplementation can lead to an undesired imbalance between calcium and alkalinity. Indeed, it is this imbalance that often lies at the heart of poor coral health.

Let’s examine how a calcium reactor works, in a bit more detail. The reactor itself is nothing more than a chamber that holds some sort of calcium-based substrate. At normal aquarium pH (7.9-8.3), this substrate is insoluable. At lower pH, however, the substrate dissolves, thereby freeing calcium and alkalinity ions so that they can be dripped back to the aquarium.

The pH inside the reactor is lowered by injecting a small amount of carbon dioxide gas into the water as it circulates through the chamber. In most cases, a pH between 6.5-6.7 will yield the best results. Too low a pH will quickly turn the substrate to mush, and too high a pH won’t dissolve the media.

As this acidic water circulates through the reactor at a very fast rate, two other processes take place. Raw aquarium water is slowly fed into the reactor, and at the same time, calcium and alkalinity-rich water is dripped back to the aquarium. These two processes take place at the same rate, so that the reactor doesn’t overflow or run dry. The rate of flow is very, very slow – just a few drips per second in most cases.

That is how a calcium reactor works, in a nut shell. Now let’s examine some of the individual components that you’ll need to become familiar with if you’re going to purchase and operate a calcium reactor. Most retailers sell the reactor (which will include the pump and associated tubing and valves) separately from a gas cylinder kit (which will include the regulator, solenoid, and needle valve).

The Tank: The tank is a generic cylinder designed to hold pressurized gas. If you’ve got the space, get as large a tank as possible so that you won’t have to refill it as often. Avoid the tiny bottles if you can - they empty too quickly. A 5 or 10 lb. tank will usually last for several months, or up to a year or more.

The Regulator: This is the set of guages and valves that screw onto the cylinder to regulate the flow of carbon dioxide to the reactor. This is not a piece of equipment that you want to skimp on. Regulators of dubious quality are frequently sold in kits and they can lead to all sorts of problems. Spend the extra money to get the highest quality regulator available – this will make tuning and adjustment much more easy and consistent. The regulator assembly typically consists of a single control knob and two pressure guages. The first guage measures the tank pressure and the second guage measures the pressure supplied to the needle valve.

The Solenoid: Not entirely necessary, but a nice safeguard feature and one that is included in most regulator kits. The solenoid is an electronic controller that will stop the flow of carbon dioxide if the power goes out. This prevents the reactor from filling up with gas, which could then escape back to the aquarium.

The Needle Valve: Most regulator kits also include some sort of needle valve, which is used to control the amount of carbon dioxide to the reactor. Like the regulator, this is not a part you want to skimp on. Low quality needle valves are difficult to adjust and will yield inconsistent results.

The Reactor: There are a few different features to look for when it comes to the reactor chamber itself. You want a reactor with a large lid that is easy to open and close – this allows for quick and easy service. The chamber body should be built from thick acrylic, and include threaded fittings that will not leak or break. A clean and organized design is a plus.

The Pump: The water pump that drives the reactor is a critical piece of equipment. You’ll want to find a reactor that includes a reliable, high quality pump. Inexpensive pumps run loud and they can fail after months of use. Check to be sure that the pump can be quickly removed for service, if necessary.

The Feed Pump: Most reactors also benefit from the use of a feed pump, which supplies the reactor with a steady flow of water from the sump. An inexpensive submersible pump like a Maxi Jet 600 can be used, or a more expensive dosing pump also works well.

The Bubble Counter: A transparent tube that allows you to visually inspect the rate of gas flow into the reactor, usually measured in bubbles per minute.

The Check Valve: It is important to place a check valve between the regulator and the reactor, so that water isn’t allowed to enter the regulator or the gas tank. If the reactor you purchase doesn’t include a check valve, be sure and purchase one separately.

The Effluent Valve: The effluent valve controls the flow of calcium and alkalinity-rich water back to the aquarium. Inexpensive effluent valves are hard to adjust for consistent flow, so find a reactor that doesn’t skimp on quality here.

Extras and Add-ons: A pH probe holder is a nice feature to look for. Adding a pH probe will allow you to quickly inspect the reactor’s internal pH, which makes for precise adjustment. Some reactors come with a second stage, which helps to absorb extra carbon dioxide gas before it can escape back to the aquarium.

Tuning and Adjustibility: The ability to tune and adjust the reactor for consistent performance is, in my opinion, the most important feature to look for. The reason you’re investing big bucks for this piece of equipment is for peace of mind and reduced maintenance/hassle – a finicky reactor will only drive you mad.

This article was written by Jason Kim, owner and founder of AquaC, Inc.