Yes you can use kalk
in small tanks. The biggest thing is to monitor your alkinity and calcium levels for a period of time after starting the drip to make sure you dont get those level out of balance.
Ideally you should setup the drip to replace the evaporation so you only drip a small amount per day.
What I do is take 1tsp of pickling lime and mix it with 1 gal
water. Shake well and then let sit for 24 hours. THen drip off the clear water and leave the white power that settles out of solution at the bottom.