Let me clarify: if your tank has stopped drawing, say, 1/2 tsp of Kent Turbo Calcium beads per week (for example), and has started drawing a whole teaspoon per week, you're on your way to needing this. If you reach the point where you are dosing daily, you definitely can use this.
When I talk about amount of kalkwasser (limewater in the ATO) I use, that is rated in quarts or gallons per day, because that is your topoff: my 54 gallon tank, for example, evaporates a gallon a day (a high evaporation rate is good in a tank)---and I was replacing that gallon daily with ro/di. Now I simply add lime to my ATO reservoir, so I am in effect throwing in 1 whole gallon of limewater (kalkwasser, in German) per day instead of just plain fresh water.
Understand, you don't need a stirrer or mixing pump for this: what dissolves in the first stir is all that will ever dissolve. The residue can lie harmlessly at the bottom of your ATO, once it settles, (and do not use the limewater UNTIL the reservoir has settled as much as it will!)---and once you add more ro/di water to that reservoir, the residue will mix into that water and become limewater in exactly the right strength: any little molecule of lime that can't bind itself to some water molecule has to fall to the bottom again and wait out the NEXT addition of ro/di. Think of it as dates to the big dance: if you don't get asked, [get a water molecule of your own] you have to wait for a new batch to get poured in.
Mrs. Wages' Pickling Lime has very little in it that is not lime---but after a number of months, like every half year, hosing out your topoff reservoir will remove anything that won't dissolve.
You can buy kalk by the jar, but it's way pricier. One of the beauties of using the pickling lime (which is actually packaged for making pickles, in canning) is that it is incredibly cheap, but real high quality all the same.
Understand, there IS lime [calcium] in your salt mix---and this is the big difference (besides price) between reef salt and fish-only salt. Reef salt has enough calcium to sustain corals and feed them a little. It also has some other traces corals need. SO you still need your water changes to keep those traces supplied: they're far too futzy (and expensive) to add any other way, as a home hobbyist. BUT once corals suck all the calcium out of the salt mix, they need more, and more. Not only that, your fish need it---not near as much, but some, just as you do, as a living creature. Calcium is part of the muscle power cycle (calcium/lactic acid) and total lack of calcium is serious stuff. Calcium is part of the skeleton. Bone can weaken. Muscle stops working, including the heart; snail shells dissolve little holes with the snails still living in them, poor things. So if you have stony coral, and it is enclosed in a glass box sucking up all available calcium---you cannot neglect to supplement it by some means. A living reef is incredibly hungry for this stuff, because, remember, the whole bony structure of the reef itself is built out of calcium, and it is incredibly efficient at sucking it up.
People fuss a lot about 'how do I feed my corals'---and if it's a stony, you primarily give it 2 things: strong light that powers its internal zooxanthellae (photosynthetic cells) to produce sugars for its energy, and calcium that powers its expansion and contraction, its ability to suck water, and enables it to divide and build skeleton. Any shrimp you also feed it is gravy: but these 2 things it has to have. And it will rob your fish and inverts to get this one.