You can use kalk to keep calcium and alk up instead of dosing 2-part (for relatively low stony coral and clam bioload systems), or you can use it in addition to 2-part. Kalk will raise your pH more, which is a positive feature for many. There are several different ways to dose kalk. The simplest and cheapest way is just dissolving as much as possible in your ATO water and letting it dose with evaporation. This is handy, but can also be messy due to undissolved calcium hydroxide and a constantly forming skin of calcium carbonate where the water contacts the air. You also have to make sure your ATO pump and tubing are robust enough to handle this stuff without clogging or corroding. If you need a siphon break for your ATO reservoir, I wouldn't recommend using kalk this way at all. The other way is to use a kalk reactor of some sort. Pump the clean ATO water through the reactor, which typically has a stirrer to keep the solution saturated, and then out to your sump. I've found this is the most stable method for me.
