Kalk will ruin your ATO pump but for someone like me it's significantly cheaper to buy a cheapie pump replacement every few years than a Kalk stirrer (especially since my tank is a whopping 29g).
Kalk is great for maintaining stabil params, it's best hidden benefit is keeping pH stable around 8.3 which is ideal.
Start small with 1 TSP per gallon of ATO water, and test daily to see the swings, and adjust upwards from there. Maximum amount of Kalk you can use in a gallon is 2 TSP, any more than 2 TSP per gallon and it will precipitate out unless you add vinegar to help more Kalk absorb into the water -- Which is something I do not have experience with.
My tank which is again, 29g, and only filled with frags has a 3g ATO. Currently with my 25'ish frags (mostly SPS, 2 acans, 2 favia, 6 Zoa's) my tank is sustained relatively well with only 4 TSP of Kalk in 3 gallons of ATO water. To this I can and will dose about 20ml of Alk a week. Within a week of Kalk dosing most of the algae's in my tank had died out. I *believe* this is due to the more stable pH, as a pH of 8+ supposedly makes it harder for Algae to survive due to some magic I do not know the chemistry / biological aspects of.
Hope this helps!