Thanks so much. But kalk seems complicated. Please do explain how and what it does
Kalk (Calcium Hydroxide) is 2-part, but in one part, basically. It's simpler to dose than 2-part because it will add calcium and carbonate in the right ratios. The easiest way to do it (and the way it was commonly done in the past) is just to take the powder, mix it into your ATO bucket and let the ATO dose it when water evaporates. Super simple and mostly effective. Combined with some water changes and maybe a trace element solution or two, this is pretty much all the dosing you need for an everything-but-acros SPS tank.
However, there are 2 problems to using ATO to dose kalk. The first is that it's caustic and hard on your ATO pump. If the pump itself is sitting in a bucket of kalkwasser, you're going to have to replace your ATO pump every 6 months, which is expensive. The solution to this is a "kalk stirrer" where you pump fresh RO/DI water into the stirrer, and the water pressure forces Kalk out. You also get a more even/predicatable amount of kalk since the saturation is constant.
The second problem with using ATO is that it's not very exact. In the summer, you're going to be evaporating more water than in the winter, and even day to day evaporation can change based on the weather, if your windows are open, etc. So you can easily overdose or underdose and Alk can swing a lot back and forth. So you can use a dosing pump. You use test kits or automated testers to tell you how much to dose, and mix the kalk to the correct concentration. Your ATO will work less often, but as long as you're dosing less kalk than evaporates every day, you're fine.
This will work up to a point, but only so much water evaporates from your tank every day. If you need to dose a gallon of Kalk a day to maintain your Calcium/Alkalinity and only 0.5 gallons evaporated that day (maybe the windows were closed), the extra water will lower salinity (and at some point, the tank will overflow all over your floor).
Once you get to that point, you need to use either 2-part or a calcium reactor (or calcium formate like All-for-Reef) to maintain calcium and alkalinity. You probably want to keep using kalkwasser and just use the other method to maintain the difference. Once you have a 2-year-old tank filled with SPS growth and coralline, you'll be there. But just kalkwasser + water changes for trace elements can take you far.
Oh, also Kalkwasser is fairly cheap (the cheapest way to maintain Ca/Alk) and it raises pH, which helps croals grow faster, which are reasons it's popular.
