One-part calcium and alkalinity supplements are relatively new in the US. They use a single liquid to add both calcium and alkalinity at one time. The reason they were not common for many years is that the bottle does not contain calcium and alkalinity. It has calcium in it, but the alkalinity is not included. Instead, it includes an organic molecule that gets metabolized into alkalinity by bacteria in the tank. An example of a one-part supplement is Fauna Marin Ready2Reef. Because the alkalinity is not directly included, you need to wait several hours for it to show up on test results. Keep this in mind when making changes to your parameters.

One-part supplements should be added daily, and because of this, many will automate their addition with a dosing pump.

Ultimately, one-part calcium and alkalinity additives are the simplest and easiest way to add calcium and alkalinity, but they are also the most expensive in the long run because the price of the chemicals per the amount of calcium and alkalinity added is the highest of all the supplementation methods. Some one-part supplements may also include trace elements, but these are also at an additional cost—making them even more expensive in the long run. One-part supplements also don’t work well on tanks with very high calcium and alkalinity consumption because sometimes the bacteria struggle to metabolize large amounts of the additive into alkalinity fast enough. This makes one-part additives an excellent choice for small aquariums, low-demand soft coral tanks, and for aquarists who prefer convenience over long-term savings.

Two-part supplements are the most popular way to add calcium and alkalinity to a reef aquarium. Two-part supplements consist of two solutions, one that increases calcium and one that increases alkalinity. Most brands mix the two solutions at concentrations where you should be adding approximately equal parts of each solution. Some popular two-part solutions include Two Little Fishies C-Balance, Brightwell Reef Code A and Reef Code B, and Fauna Marin Balling Light CA used along with Fauna Marin Balling Light KH. Our favorite two-part with trace elements included is C-Balance.

Two-part is more affordable than one-part solutions per the amount of calcium and alkalinity added, and just like some one-part solutions, you can purchase two-part additives with trace elements added in at a bit of a price premium.

There are some drawbacks to two-part solutions. Two-part needs daily additions just like all of the other calcium and alkalinity methods, but automating the dosing requires two dosing pump heads instead of the one dosing pump head needed for kalkwasser or one-part dosing.

Two-part is also more expensive in the long run per the amount of calcium and alkalinity added when compared to kalkwasser or a calcium reactor.

Additionally, two-part causes the aquarium's salinity to rise over time. This is because the two ionic solutions that are added are typically CaCl and NaHCO3. The Ca in the CaCl is calcium, and the HCO3 in the NaHCO3 is carbonate (alkalinity). Adding these solutions does increase calcium and alkalinity, but it also adds extra NaCl (salt) in the process. If you are adding very large amounts of a two-part solution, then this can lead to problems of gradually increasing salinity.

An additional problem is that only NaCl is increased, and none of the major or minor elements are added with the regular salt. This would be like trying to keep a saltwater aquarium using table salt instead of sea salt. Ultimately, this ionic imbalance is not good for the health of fish and corals. Regular water changes can help to correct this imbalance. Thankfully, the salinity-raising effect is typically only noticeable if you are using a lot of two-part. Because of the relatively high price of two-part and the salinity shift problems, aquarists with high calcium demands typically choose a different supplementation method. However, two-part is perfect for the vast majority of reef aquariums with a moderate calcium demand and regular partial water changes.

Kalkwasser is perhaps the king of all aquarium supplements. It is an extremely affordable powder made of CaOH. It is available from a variety of manufacturers, including Brightwell Aquatics and Seachem. This one additive increases calcium and alkalinity in the exact ratio found in coral skeletons and also boosts pH at the same time.

There are only a couple of drawbacks to using kalkwasser. Kalkwasser does not dissolve well. There is no way to concentrate the solution to more than two teaspoons per gallon of water. This means that in some aquariums with very high calcium and alkalinity demand, you will not be able to add enough kalkwasser to keep up with the calcification demand of the animals. You can only add as much kalkwasser as your tank evaporates. Adding more will slowly increase the aquarium’s water level until the tank overflows. While kalkwasser alone can work for 95% of reef aquariums, if you really know what you are doing and are growing coral really fast, then kalkwasser alone will not work for you.

Another drawback to kalkwasser is that it increases pH. You may be thinking this was an advantage of kalkwasser, and you would be correct thinking that increasing pH is good for coral health and growth. But a sudden drastic increase in pH can absolutely kill everything in your tank. Because of this, kalkwasser needs to be dosed slowly throughout the day.

In most cases, aquarists mix up a batch of kalkwasser and then use a programmable dosing pump such as a CoralVue Hydros Minnow Controller to break up the dose into small hourly doses automatically. You should never manually pour kalkwasser directly into your aquarium.

The most common way to automate kalkwasser dosing is to use a kalkwasser reactor such as the IceCap Kalkwasser Reactor in conjunction with a dosing pump. The dosing pump draws RO water from your ATO container and pumps it into the kalkwasser reactor. The reactor has an overflow port that slowly overflows into the aquarium sump as the new water is pumped in. Inside the reactor is a whole bunch of Kalkwasser powder and a magnetic stirrer at the bottom. The magnetic stirrer turns on periodically to mix the powder into the water inside the reactor.

The advantage of the kalkwasser reactor setup is that you can add a lot of kalkwasser powder to the reactor and only come back to add more every couple of weeks. In many cases, you can add enough powder to make 20 or even 50 gallons of kalkwasser. Most people have nowhere to store this much kalkwasser by their tank, and as a consequence, if they are manually mixing, they can only mix a few gallons at a time and have to mix new batches of Kalkwasser several times per week. A kalkwasser reactor is not necessary, but it does make using kalkwasser a lot easier.

Kalkwasser is best for intermediate to advanced hobbyists who are confident setting up the reactor and value the long-term savings of the very affordable powder over bottled two-part or one-part supplements. Kalkwasser is also very commonly used in conjunction with other supplementation methods on high-demand aquariums where kalkwasser alone cannot meet the calcification demand. When kalkwasser is used in conjunction with other methods, kalkwasser saves money on two-part and one-part solutions because less of the more expensive solution is needed. Kalkwasser used in conjunction with the two-part method reduces the salinity increase, and the pH-increasing effect of kalkwasser helps to mitigate the pH-decreasing effect of a calcium reactor.

Calcium Reactors, such as the Reef Octopus Calcium Reactor, are the dream of nerdy advanced marine aquarists and the nightmare of the beginner. Calcium reactors work by filling a reactor chamber with old dead coral skeletons and then recirculating the water while a small amount of CO2 gas is bubbled in. The CO2 gas slowly melts the dead coral skeletons, and then a separate pump flows water through the reactor, delivering the elements from the melted coral skeletons to the aquarium. There are some big advantages with calcium reactors. Calcium reactors dissolve actual coral skeletons, so they add not just calcium and alkalinity but all of the other major and minor elements found in coral skeletons in the exact ratio from coral skeletons. The calcium reactor media is also very affordable, coming second only to kalkwasser in the cost per calcium and alkalinity added. These benefits make calcium reactors the best choice for very large aquariums or aquariums with very high calcification rates. Calcium reactors are what nearly all public reef aquariums and professional coral grow-out facilities use to supplement calcium and alkalinity.

There are some drawbacks to calcium reactors that make them not the ideal choice for the average home aquarist. Calcium reactors are extremely complicated, difficult, and expensive to set up. To set up a calcium reactor you need the reactor itself, a CO2 regulator and CO2 tank, a specialized supply pump with variable speed capability such as the CoralVue Hydros Sole Dosing Pump, and a pH controller to monitor the amount of CO2 added to the reactor chamber. This is more money and time than most aquarists want to invest in calcium and alkalinity additions (though for some the toys are half the fun). Calcium reactors also tend to lower the pH of the aquarium. This is because some amount of the CO2 gas added to the reaction chamber makes its way back into the aquarium. CO2 is an acid that lowers pH and slows calcification. Because of this, it is very common for tanks with high coral calcification to use kalkwasser in conjunction with the calcium reactor to counteract the pH-reducing effects. This only serves to complicate the system further, making this a method only for advanced aquarists, zoos, and aquaculture facilities.