For many years aquarists around the world have been using kalkwasser (limewater) or calcium hydroxide (CaOH to maintain high pH and add addition calcium to their aquarium.  Indeed, in more recent times there has been a push to solve many of the reefing issues with new technology, e.g. LED lighting, aquarium controllers, and any number of reactors.  However, sometimes the simplest things work best with little to no adjustment after being dialed in.  This is exactly where using kalkwasser in your auto top-off (ATO) water can solve not only your low pH values, but your alkalinity issues as well.  Nevertheless, it will take a little work to dial it in, yet once you have it, you will be able to maintain your pH levels above 8.00 and have your alkalinity only drop 0.1 dKH over several days. In addition to this, dosing Kalk has been found to help with Phosphate sequestration as well as buffer against reef hobby pagues like Cyanobacteria ( red/green/black slime) and even help in preventing dinoflagellate (dinos) outbreaks.

The first step is to measure you dKH over a few days and understand what your tank uses on a daily basis.  Depending on your tank type, i.e. mixed reef, small polyp stony (SPS) or large polyp stony (LPS) dominant, softies, or non-photosynthetic, your tank may use varying amounts of carbonate.  Mixed reefs and SPS dominant tanks have a tendency to use more because they deposit a substantial amounts of calcium carbonate throughout the day.  While softies will use less, they will still consume and deposit calcium carbonate in little spiky rice granules called sclarites.  In addition non-photosynthetic corals will also consume calcium carbonate and build their skeleton the more they are fed and thus grow.

dKH, Kalkwasser, and Alkalinity


Kalkwasser or calcium hydroxide, when dissolved in water will also create calcium carbonate.  Without going into too much chemistry, carbon dioxide (CO2) dissolves readily into water forming carbonic acid.  This is one of the reasons scientist around the world are concerned with climate change, higher CO2 in the atmosphere will lead to higher dissolved CO2 in the ocean and thus lead to ocean acidification.  However, when carbonic acid in water comes into calcium hydroxide, it will precipitate out in your tank and basically make limestone.  This is why it is so important to not only make a kalwasser solution that you will use within a few days but to also make sure you cover your kalkwasser solution.  The more the kalkwasser solution has the opportunity to mix with aqueous CO2 (CO2 dissolved in water), the faster it will precipitate out of solution and the less effective it will become.  For this reason, it is also important not to aerate your kalkwasser solution.  Nevertheless, as you continue to add kalwasser to your aquarium, it will still leave some calcium carbonate in solution which is what contributes to your alkalinity.  Think of it in terms of water hardness, when your tap water is hard, you buy a water softener to lower your water hardness so calcium carbonate does not build up in your pipes, clog your faucets, or “stain” your dishware.  Kalkwasser is basically the opposite; you are adding “hardness” back into your reverse osmosis deionized (RODI) water.  Therefore, it is very important to continually measure your alkalinity while you dial in your tanks carbonate usage and thus kalkwasser dosage.

Dosing Kalkwasser

In many cases most manufactures will suggest starting with 1 teaspoon of kalkwasser or 3 grams per gallon of water.  However, unless you are running a well-established SPS dominant aquarium with a massive bioload, I would seriously doubt your aquarium will use or need this much to start with.  In many cases, using too much of anything is bad and in the case of kalkwasser, this is definitely the case.  Adding too much kalkwasser will not only cause a pH spike, it will also cause an alkalinity spike that will lead to a bleaching response in your corals.  All too often the miss use or over use of kalkwasser has led to many aquarists “burning” their corals.

With everything in this hobby, it is best to not “cowboy it”, and rush things; everything that happens quickly in this hobby is almost always bad!  Nevertheless, it is best to start with ½ a teaspoon of kalkwasser per 5 gallons and work your way up from there.  Depending on the size and configuration of your tank, i.e. 20 gallon vs 120 gallon tank or covered vs open top tank, you can expect to use the 5 gallons of top-off water in 3 days or over a week.  During this time you will want to test your alkalinity at the same time every day and maintain a log book of the alkalinity rise or drop over time.  Keep increasing the amount of kalkwasser to your ATO water each time you refill it until you notice a drop of 0.1 dKH over 2-3 day period (0.5, 1.0, 1.5, etc. each time you refill your reservoir).  It is also important to rinse out the precipitate or sediment at the bottom or you reservoir before refilling it.  If you don’t, it will continue to build up and will throw off the accuracy of your test results.  If you see a rise over a two to three day period, knock back the amount of kalkwasser being added to your ATO reservoir.  It will not cause any short-term damage to your corals by having a low pH (7.7) or low dKH (7.0), however, it will if you have a high pH (>8.45) or high dKH (10.0).  This will cause the classic “burning” where your corals will lose coral, “brown out”, and or bleach.  Low pH in many cases will not kill a coral and will only lead to a lower calcification rate; however, while high pH is characteristic of the calicoblastic layer where coral forms their skeleton, it can have deleterious effects over a long period of time.

One final word of advise, when your ATO reservoir is low, and you have had substantial evaporation.  It is best not to let your tank refill with a gallon or two of freshly mixed kalkwasser.  If this happens, added the equivalent amount of water lost to evaporation of a 1:10 ratio of kalkwasser to RO, i.e. if your ATO has stopped working because it ran out of reservoir water, and you have lost 1/2 a gallon of water to evaporation, add a 1/2 a gallon of 1:10 kalkwasser solution back into your tank before you reset your ATO.  This will allow you to compensate for the water lost while your ATO was not “topping off” and not cause a huge alkalinity/ pH spike at the same time when your ATO turns back on

Kalkwasser’s beneficial side effects

Over the years, it has been noted that kalkwasser has a few side benefits that are easy for the average reef aquarist to leverage as they strive to maintain a healthy reef tank.  The first of these is the ability at the molecular level for kalkwasser to bind phosphate out of your reef tanks’ water column.  This binding removes the phosphate and makes it unavailable to things like algae, cyano, and dinoflagellates, all of which can cause an unsightly patch in your reef, or worse, effect the health of your corals.  While this process won’t remove huge amounts of phosphate like say GFO or ROWAPHOS, in most tanks it will be enough to help maintain low phosphate (po4) levels over a long period of time.

Akin to this phosphate binding ability, Kalk can be a great weapon against Dinoflagellates and c

yano directly by way of its pH raising ability.  It has been fairly well documented that an elevated pH of around 8.4-8.6 for a period can help create an environment that is unfriendly to these 2 reef tank pests.  It remains unseen if long term Kalk can remedy and or prevent dino/cyano infestations from occurring, but time will tell if this is the case.  In the meantime Kalk appears to be at least another tool that reefkeepers can enlist in the fight against these stubborn reef pests.

Dosing other methods in conjunction with kalwasser

Even though your dKH will drop over several days or if you dial it in really well, over weeks, doing your regular water change will help bump it back up again.  If you use a two part dosing method, you will use substantially less of the alkalinity component now that you are adding it through the kalkwasser.  Again, you will need to test your alkalinity while dialing in the two part after dialing in the kalkwasser.   Nevertheless, you will still need to maintain dosing your trace elements, kalkwasser is only adding calcium and not the other trace elements such as magnesium, strontium, potassium, etc.  It is always good to check your alkalinity at least once a week after dialing it in to help ensure you are not slowly rising your dKH or your corals are using more calcium carbonate facilitating the need to add more kalkwasser to your ATO water.


Happy reefing!