Calcium levels aren't nearly as big of a concern if all you have is fish. High calcium and alkalinity levels are needed for many corals, as they use both to build their inner structure. I'm sure you've seen a skeleton of a dead acropora or other sps coral. That nice white structure is made mostly of calcium and KH (alkalinity.)
As for adding a bunch of an additive at a time. It's not recommended at all. Assuming your talking about a calcium or KH additive other than lime water - additions should be made in smaller doses more frequently rather than in one big dose all at once. Especially with KH buffers, as they have a huge impact on PH.
Although I don't fully understand it, the easy explanation is that calcium and KH are a delicate balance with eachother. One affects the other. This is where the 2 part additives and lime water come in. They are "ionically" ballanced. Add equal parts of a 2 part additive to raise both equally. If your system is out of balance you can add more of one than the other. But in most cases, what's really needed is to add equal parts of both to bring them back into balance. And obviously with lime water you just drip it, and adjust the balance with either a 2 part or other sperate additives. But unless things are severely out of wack, I would say just keep driping the kalk, try to replace 100% of evaporation with it initially untill the levels come up to where they should be, and most likely they will balance eachother out. Otherwise it'll be a calcium/alkalinity roller coaster ride.
After the levels come up to where you want them you can probably back off the lime water solution. I personally like to keep the lime water saturated, and just drip less of it, as I also use an auto top off system. But you can also lessen the saturation of the lime water and continue replacing 100% of evaporation with the less concentrated solution. Either way accomplishes the same thing.
For those with heavily sps dominated tanks, even saturated lime water won't keep up with calcium consumption. This is were reactors come in. They use CO2 to lower PH in a reaction chamber that is filled with crushed coral. The crushed coral disolves in the low PH, releasing calcium, KH, and many other trace elements.
Most of us strive for calcium around 450 and KH of at least 10dkh. Give or take. If you have no hard corals you can get by with lower calcium levels. For those with SPS dominated tanks, 450 is probably the lowest they want to go(?)
If you feel your calcium is low, probably best not to go add a bunch of just a calcium additive. Drip the lime water, and give it a week or two. It'll bring up your calcium and KH in balance to where you want them.
. But I think its easier to find during canning season.
