What is Kalwasser? And how to dose it?

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Reefing newb
What is Kalkwasser? And how do I dose it?

By Robert M. Metelsky
Author of the book Simplified Reefkeeping, 3rd ed. available at simplifiedreefkeeping.com


Because the tank is exposed to air, and strong pumps are circulating the water throughout the filtering system (overflow pipes, drip plate, prefilter, and protein skimmer), you will get a significant amount of evaporation. In fact, the better your skimmer and the stronger your water pump (both desirable features), the more evaporation you will get. You will need to replace the evaporated water regularly. An important reminder for the new hobbyist is that the water evaporates, not the salt. Do not add salt mix with the make-up water. The result will be a higher salinity than is desirable.

Adding make-up water provides a good opportunity to replenish much-needed calcium, which gets depleted rapidly in an enclosed reef system. This vital element is used by virtually all living creatures. Some of it also gets removed by protein skimming. In my opinion the best calcium additive is “Kalkwasser,†which is calcium hydroxide. It is added on a regular basis by mixing it with the purified water being added to compensate for evaporation. These regular additions of calcium hydroxide also keep the pH elevated to the desired 8.2 to 8.4 level.

Kalkwasser is a German word. Literally, it means “lime water.†Kalkwasser is a trade name for calcium hydroxide. The terms “Kalkwasser,†“limewater,†and “calcium hydroxide†all mean the same thing in this hobby.

The water you use to replace what has evaporated will be called “make-up water.†It is extremely important to use purified tap water mixed with calcium hydroxide (a.k.a. Kalkwasser, a.k.a. limewater) for the make-up water! Do not, I repeat, do not, use regular tap water or anything else for make-up water! This is asking for trouble.

As I have stated from the beginning, nothing will ensure your success more than the quality of your water. Once you have made the investment of a water purifying system and have started the reef with purified tap water, the reef will be accustomed to that quality of water. It would be extremely foolish to try to cut corners here. This is the last place to skimp. In fact, it would be inviting disaster by possibly introducing impurities (metals, silicates, phosphates, etc.) that are harmful and troublesome (hard to remove) into the pristine environment that we have tried so hard to create.

When to add make-up water:

Add the Kalkwasser within a day after you mix it; it gradually loses effectiveness after it is mixed Watch the water in the sump! This is where you will see the change in water level. Once you have established the “working water level†in the sump, mark it on the side of the sump box, with magic marker. This will give a quick visual reference as to the height of water that is normally in the system. As evaporation occurs, watch this mark. When the level goes down by 3 to 5 gallons, or gets close to the top of the outlet for the pump, you need to add make-up water. Mix the water no more than one day before you add it to the tank; it starts to lose effectiveness right after it has been mixed. It will have the highest concentration of available calcium just after the sediment settles out of the solution.

On a smaller tank (even a 55-gallon), 5 gallons of high-powered make-up water must be used with caution! Kalkwasser has an extremely high pH. Pay close attention to the drip/dosing flow of water, to be certain that it is administered very slowly. For a 55-gallon tank, you should take a 48-hour period to administer 5 gallons of make-up water. Make sure you test-run your drip method, to be sure that it introduces the desired amount of make-up water over the correct period of time. Adding Kalkwasser too fast will cause pH shock, which can be fatal or, at the least, unnecessarily stressful to the livestock. Take the recommended precautions and do not let this happen!

On larger tanks, 125 gallons and up, 5 gallons of make-up water will not have as much of an effect as it will in smaller tanks. For a 125-gallon tank, the Kalkwasser can be added at the rate of approximately 5 gallons in 8 to 12 hours. In a 200-gallon or larger tank, the 5 gallons can be added without any clamping system, allowing the airline tube to empty the 5-gallon bucket unrestricted. This will take less than 1 hour.

Time of day to add:

Another suggestion is to add the Kalkwasser mix when the tank lights go out, or (ideally) first thing in the morning. While the lights are off, the pH drops, reaching its lowest level the next day just before the lights come back on. If you add the Kalkwasser during this reef “night,†the effect of raising the pH will not be as significant as it would be during lighted hours.

There may be some questions and concerns about adding 5 gallons of Kalkwasser all at once. Yes, some critics may be correct that adding smaller amounts more frequently would be a less risky, less stressful, and more natural approach. However, I have used my method on tanks from 55 to 200 gallons, with no adverse effects, and I have not lost one creature due to pH shock. You do have to be careful on smaller tanks, but once you get familiar with this system, I’m sure you will find it to be very practical: (1) you will add make-up water less frequently, and (2) on larger tanks (125 gallons and up), you can add 5 gallons of make-up water at a time, which is a significant, convenient, easily measurable amount of water to add.

Do not mix with an airstone; this will add carbon dioxide and oxygen, which will reduce the effectiveness of the calcium hydroxide and defeat its purpose!

Benefits of adding Kalkwasser:

You may be interested in why it is so important to add Kalkwasser. Some of the benefits are:

* It adds calcium that is needed by most of the creatures in the reef.
* It encourages the growth of pink and purple coralline algae.
* It keeps the pH elevated. By adding Kalkwasser on a regular basis (make-up water) and doing water changes every 2 to 3 weeks, I have found my pH to be consistently between 8.2 and 8.4. Keeping the pH at this level makes it less likely that micro-algae will become a problem.
* The reef just seems to love Kalkwasser.
* There are many more scientific and chemical reactions that are beneficial. Take my word for it: adding Kalkwasser on a regular basis is one of the most beneficial procedures for maintaining a healthy reef and desirable water chemistry.


When Calcium Hydroxide solution (Kalkwasser) is slowly dripped into your aquarium, it captures free Carbon Dioxide present in the tank water and converts it to Bicarbonate ions (which is a good thing), like this:

Ca++ + 2(OH-) + 2(CO2) <==> Ca++ + 2(HCO3-)
If you drip too fast or if there is not enough Carbon Dioxide available in the water, your shiny new Bicarbonate ions will be converted to Carbonate ions (a bad thing), like this:

Ca++ + 2(OH-) + 2(HCO3-) <==> Ca++ + 2(CO3--) + 2 H2O
The Carbonate ions formed will make the Ca++ you are trying to add to your tank get wasted by the useless precipitation of Calcium Carbonate -- the white stuff you are seeing.

So, too rapid addition of Kalk may actually cause the Calcium and Alkalinity in your tank to go DOWN instead of UP (a bad thing), like this:

Ca++ + 2(HCO3-) + Ca++ + 2(OH-) <==> 2 CaCO3 + 2 H2O
In the above reaction, a Calcium ion and two Bicarbonate ions from the aquarium combine to form solid calcium carbonate -- the white stuff you are getting in your tank, which is really just a kind of sand.

This can happen even with a slow drip of Kalk if there is not enough CO2 in your water -- something you can't easily control.

To avoid this, try mixing and adding your Kalkwasser like this: pour 15ml of 5% Acetic Acid (or ordinary Distilled White Vinegar from the grocery store -- same thing) into a 1 liter (1 quart) container. Dissolve 1/2 teaspoon of lab-grade Ca(OH)2 (or commercial Kalkwasser mix) in the Acetic Acid, and then dilute to 1 liter (1 quart) volume with either RO/DI water, or even tank water.

15 ml is more Vinegar than some people are comfortable with, but I use it constantly with no problems. There should be no sediment in the mixture, or just a little bit at most. You can let the sediment settle out if you don't like the white flakes in your tank. I just drip the liquid and the sediment both into my tank.

Dissolving the Kalk powder in the Vinegar first will accomplish several very good things.

First, it will get more Calcium ions (Ca++) into the solution because you are dissolving the Ca(OH)2 in an acid instead of water, and forming Calcium Acetate, which exists as a dissociated equilibrium of free Calcium ions and Acetate ions.

Second, the Acetic Acid (Vinegar) provides an equivalent of all the CO2 you need to avoid precipitating the newly-added Calcium ions as useless white Calcium Carbonate powder.

Third, after all the cool Calcium ion chemistry is over, the leftover Acetate ions from the broken-down Vinegar leaves you with free organic Carbon in the water that feeds the bacteria in your tank so that it converts more poisonous Nitrates to NO2 gas (a very good thing).

Adding Vinegar in Kalkwasser is one of the few win-win situations for reefers -- it has a great up side and I've yet to encounter a down side to doing it. I don't know why so few reefers do it -- lack of understanding of the chemistry behind it maybe -- but a lot more are starting now that some respected reef writers have discovered it and have recommended it and even written up detailed instructions for it.

By the way, you should check your pH before and after you do this the first few times to make sure it is not affected by the process. It should not be a problem. Also, if you don't already have them, get and learn to use Salifert test kits for Calcium, Alkalinity, and Magnesium. The levels of all of these are related and affected by dripping Kalk.

The advice not to mix the Kalk too much is right on target. As the equations I posted show, if you are not using any Vinegar in the mix you certainly DO need to avoid mixing the Kalk powder with CO2 until AFTER it's dripped into the tank.

Look again at the equations. Both the GOOD reaction that liberates useful Calcium ions (Ca++) into your tank, and the BAD reaction that wastes the Calcium ions as solid Calcium Carbonate (CaCO3), require the addition of different amounts of CO2.

The problem with allowing the CO2 reaction to occur as you mix your Kalk in a container without Vinegar is that you are mixing it in a comparatively small quantity of water that contains only a very limited amount of CO2. So, instead of having enough CO2 to go all the way to useful Bicarbonate ions (HCO3-, or Alkalinity) as you want it to, the chemical reaction stops at the undesirable Carbonate ion (CO3--). This is what sets the stage for the Calcium ions to precipitate out as useless solid Calcium Carbonate while still in the mixing container -- the Kalk mixes with SOME CO2, but not with ENOUGH CO2.


If your mixing container contained ample CO2, i.e., enough to fully react all the Kalkwasser as Bicarbonate ions, you would indeed then want to do all the mixing with CO2 right there in the container. It's only because there's not enough CO2 present in the small container that you need to limit the mixing to avoid precipitation.

So, you've actually hit on one good reason WHY we want to add the Vinegar to the mixing container -- the Acetic Acid in the Vinegar provides the equivalent of enough CO2 to take the reaction all the way to the desired end products for which we drip Kalk -- that is, to produce free Calcium ions and free Bicarbonate ions in solution in our tanks.

By using enough Vinegar, we can mix to our heart's content right in the container, and don't need to worry as much about dripping the solution slowly into the tank. We have already driven the reaction all the way to the desired outcome, and provided ample CO2 to produce a balance of free Calcium ions and free Bicarbonate ions. This, incidentally, is why Kalk is said to be "ionically balanced" - it produces a one-to-one balance between Calcium and Alkalinity.

By the way, the "stoichiometric" amount of Vinegar, i.e., that amount that provides the exact equivalent of enough CO2 to react all the Kalk powder to Calcium and Bicarbonate, turns out to be about 25ml of 5% Acetic Acid per liter of saturated (0.02 moles/liter or 1.5 grams/liter) aqueous Calcium Hydroxide solution (Kalkwasser). I've used 30ml of Vinegar to a 1/2 teaspoon of Ca(OH)2 per liter of mix without any problem, but recommend about 15ml to those new to using Vinegar. This means that you still need to go easy on the stirring, because we are not providing quite enough equivalent CO2 to avoid SOME Carbonate ion formation if we get carried away with the mixing.

As to your question about using Vitamin C (L-Ascorbic Acid, or L-3-Ketothreohexuronic Acid Lactone) instead of Acetic Acid, its chemical formula is C6H8O6, compared to C2H4O2 (commonly written CH3COOH) for Acetic Acid. The real story for us here though is in the atomic STRUCTURE of the two, rather than just in their FORMULAS.

While Acetic Acid is essentially just an Acetate ion (CH3CO2-) with an extra Hydrogen atom tacked on, Vitamin C has an alkene ring, 4 alcohols, and an ester in its structure. I can't say with certainty all the complex organic chemistry that dissolving L-Ascorbic Acid would kick off in one's tank, or whether it would be good or bad, but I think I'll let someone else perform that particular experiment.

If you use Acetic Acid from a chemical company, buy a concentration of 5 percent Acetic Acid in water (an aqueous solution). If you use Distilled White Vinegar from the grocery store, look for an "acidity" of 5 per cent on the label. Never use any colored or flavored or Apple Vinegars.

"Kalk" powder is either lab-grade Ca(OH)2 (Calcium Hydroxide) from a chemical company, Kalkwasser Mix from an aquarium supplier, or Pickling Lime from the grocery store. It's all about the same. Use 1/2 teaspoon of the powder per quart of solution -- maybe a bit more if you use Pickling Lime from the grocery store, which is less Calcium by weight than the other sources.

Some words of caution, especially for any reefkeeping newbies out there -- If you are using Vinegar in your Kalk mix for the first time, I'd start off with 5 ml per quart at first. If you find you still need to get more Calcium into your tank per unit Kalkwasser, work up over a couple of weeks to a max of 12-15 ml per quart (the stoichiometric amount for Carbonate as recommended by Craig Bingman), or if you are feeling brave or really need lots of extra Calcium and de-Nitrate action as I do, a max of 24-30 ml (the stoichiometric amount for Bicarbonate, which I use).

Don't go whole-hog on the Vinegar from the start until you are sure your tank needs it and will take it without experiencing radical changes in pH, Calcium, Alkalinity, Magnesium, and the health of your critters and your Nitrifying bacteria. Measure ph, Alk, Calcium, Magnesium, and Nitrates with Salifert kits as you work up to more Vinegar.

Observe your livestock, especially if you have any really delicate corals or fish, to see that they behave normally as you use more Vinegar.

My tanks have very little evaporation, so I have to use lots of Vinegar to put in lots of Calcium with very little makeup water. If your tanks have tons of evaporation, use less Vinegar per volume since you don't need such a high Calcium boost per unit of Kalk water.

As with all things reefing, go slow, observe the reaction of your tank to any changes in what you add every week, and understand what and why you are doing. I don't want anyone coming back here on the board and blaming me for their fish that died when they poured 1000ml of Glacial Acetic Acid into their 55 gallon tank!

My methods are safe for me in my tanks -- your mileage may vary, so use due care.

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