p_400536_FS19709DRegardless of species, as hard corals grow, they consume more and more calcium carbonate from the water. It’s vital that the water housing corals has enough calcium available, along with the correct measure of carbonate hardness. Without this, corals cannot properly grow and a healthy reef cannot properly form. If you have a successful reef, in that you’ve maintained the right balance of calcium, carbonate hardness and magnesium, sooner or later your ph will start to fall. The animals will be using calcium and carbonate from the water, and this will in turn lower the ph. There are a host of chemical processes that explain this, but that’s more fodder for something written by Randy-Holmes Farley. Simple fact, your ph will start to drop lower, even though your alkalinity measures at or around 8-10 dKH. Many aquarists wonder why this is happening, and to correct it, they turn to carbon dioxide air reactors or more powerful skimmers. 


As a Band-Aid, these work for a while. CO2 reactors pull co2 out of the air entering the skimmer, and the oxygen-rich air helps dilute some of the tank’s co2, raising ph. You may see the ph increase a few points here or there, but in all likelihood it will drop back down with time. A powerful skimmer will inject more air and create more turbidity, and like a co2 reactor, may temporarily raise the ph a few notches. As an aquarist, you really don’t have many options to correct this issue. You could dose like it’s nobodies’ business, but in the end that will lead to swinging ph levels, along with unbalanced calcium and carbonate hardness. Congratulations, as you have a reef that is creating a moderate calcium/carbonate demand, which means it’s growing. Now, you have to find a stable way to meet that demand. Of all the options out there, a kalkwasser stirrer may be the easiest, and most cost effective.

Ph swings and calcium demand:

kalkwasserThere are multiple ways to increase both the alkalinity and calcium value of reef aquarium water. Some are as simple as baking soda, while others implement a three-part additive. These can be dosed manually, and tailored to water test results, or they can be automated if things remain consistently stable. The short-coming is that the water chemistry in a reef aquarium changes. Corals begin growing, their demand on calcium may increase, or corals are added, fragged, etc. Each time this happens, a dosing regimen must be re-designed to meet the new demand. Automatic dosing pumps need reprogrammed, and all in all, it’s a bit of a headache.

Kalkwasser is German for “lime-water.” It’s a diluted solution of calcium hydroxide. Pure limewater is clear and colorless, with the bitter taste of calcium hydroxide. You create limewater by stirring calcium hydroxide in freshwater (preferably RO/DI water) and using only the top layer of water. The calcium hydroxide is insoluble, and you don’t want it entering your aquarium. When exposed to air, kalkwasser quickly loses its potency as co2 infiltrates from ambient air.


Some aquarists dose kalkwasser directly into their top-off water, but doing this requires fresh mixing to retain the original potency. Others use kalkwasser reactors, or stirrers, which are hermetically sealed and prevent co2 and atmospheric air from getting into the solution. The convenience of using either a reactor, or stirrer, is based on several things. One, it assumes that an auto-top off (ATO) system is being used. Not having an ATO increases the complexity and cost of using kalkwasser, if you want something simple and almost entirely automated. It also assumes that the aquarium is located in a place where the aquarist can use gravity to their advantage. If kalkwasser is not used in conjunction with an ATO, then it requires a very slow drip into the tank, best accomplished with a small dosing pump and gravity.

The easiest way to implement kalkwasser:

Kalk StirrerIt’s my opinion that the easiest way to use kalkwasser, is in conjunction with an ATO. In fact, if an ATO isn’t currently in use, it would be worth investing in one, if you have a need for kalkwasser dosing. An ATO automatically does most of the hard work involved in dosing lime-water, as it measures water loss due to evaporation and replaces that with lime-water.

Kalkwasser reactors come in two general flavors. Some are reactors that use an external pump to mix the calcium hydroxide and water, at pre-programmed intervals throughout the day. This creates a slurry of calcium hydroxide and water, which would be highly volatile if dumped into the aquarium. Because of this, an additional timer is required to make sure no lime-water is dosed until the slurry has settled. If you’re using a Neptune apex, and rather proficient with it, then the apex could handle this for you if properly programmed with a few outlets used to control the system. While this type of set-up works fine, it’s a bit of a hassle to program, and requires more resources than is really needed. Even with all the programming and forethought, there is still a chance that some of that kalkwasser slurry could enter your aquarium and cause problems.

For ease of use, safety and overall efficiency, kalkwasser stirrers offer the best bet. These systems use a set of “paddles” at the bottom of the reactor, which are constantly rotated by a shaft running down the reactor’s center. Insoluble material, such as the bulk of kalkwasser powder, sinks to the bottom and the continuous stirring makes sure a constant saturation of lime-water is present at the reactors top. When your ATO detects a water level drop, the pump within a RO/DI reservoir kicks on, filling the reactor from the bottom up, and pushes saturated lime-water into your tank, until the ATO detects the sump is full. The only fail-safe required is an apex controller or ph controller to monitor ph, and kick off the ATO pump in the event the ph goes above a safe concentration, say 8.5. Stirrers come in all shapes and sizes, and are hermetically sealed so that air cannot react with the kalkwasser mixture. After several weeks of constant dilution, the kalkwasser mixture will lose potency and need replaced. Other than that, it’s a pretty automated way to control ph drops and replenish calcium and carbonate.

Not a cure all:

barr kalk stirrer5Kalkwasser doesn’t release any bi-products into the aquarium that cause problems. It doesn’t harbor phosphates or nitrates, and there aren’t nutrients within kalkwasser that cause algae issues. The biggest drawback to kalkwasser is that it only helps maintain calcium and alkalinity, it does nothing for magnesium. This means that aquarists using kalkwasser will still need to test and dose for magnesium concentration. Luckily, magnesium depletes much slower than calcium and carbonate hardness does, and is easier to manage and continually dose. It would be very easy to program a dosing pump to supplement magnesium, and have a totally independent reef, as far as balancing the three crucial reef water elements.

Another downside is that for moderate to high demand reefs, kalkwasser may not cover all calcium and carbonate depletion. If you keep a mix of SPS and LPS corals, it’s quite possible the animals will deplete more calcium and carbonate than the kalkwasser can replenish. In this instance, it may be necessary to use a dosing pump to continuously drip kalkwasser into the tank throughout the day, or a calcium reactor may be needed to maintain calcium concentration. Dripping kalkwasser gets a little more complex than using it in conjunction with an ATO, mainly because the aquarist has to carefully measure the amount of kalkwasser needed to maintain levels, but also has to prevent high range ph and severe swings, in the event the dosing pump kicks off. Those of us with high calcium demand, may find that kalkwasser in conjunction with an ATO, along with a dosing pump adding a bit of calcium supplement works best. There are a multitude of configurations to explore.

The difference when compared to manually, or automatically dosing:

Avast-22Manually dosing calcium and carbonate becomes a chore, especially as your reef grows. Often dosing alone cannot prevent ph drops, and aquarists find themselves adding more and more carbonate to the water, just to raise the ph above 8.0. This isn’t a healthy way to maintain a reef. The goal of balancing water chemistry isn’t just bringing three crucial elements up to a particular concentration, but making sure that concentration doesn’t change; hour to hour, day to day. If your calcium drops to 300 ppm, then the tank is dosed, raising it to 430 ppm, and that cycle continues day in and day out, then likely you’re are struggling to achieve good coral health.

If your home is totally environmentally controlled, using both heating and air conditioning to maintain consistent temperature, it’s unlikely your aquarium evaporation rate changes much, from season to season. In this instance, kalkwasser dosing is a constant system that creates a measurable addition of calcium and carbonate to the tank. Although, most aquariums evaporation rate changes from season to season, if you live in an area with a measurable summer and winter. For example, my tank is in a basement fish-room and evaporates less in winter, because the outside and inside humidity is low and heating keeps the temperature consistent. In summer, the tank evaporates more, as I have to use a dehumidifier to maintain low humidity in the fish room, which accelerates the aquarium’s evaporation and also adds heat to the fish-room, keeping it a few degrees over what the HV/AC system is set to. Luckily, it’s not enough of a change to throw kalkwasser dosing off or require action on my part, but it’s worth being aware of these changes.


Kalkwasser has been around nearly as long as reef-keeping. It’s likely the cheapest way to balance calcium and alkalinity, and one of the easiest systems to implement. Any tank using only manual or auto-dosing to balance calcium and alkalinity could likely benefit from kalkwasser.

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