My 30 gallon freshwater reservoir gets 2 cups of kalk powder (saturated) and lasts a week - 10 days. (3 - 3.5 gallons a day) If you are not in need of that much, you can make a less saturated mix. If it can't keep up with the need, you can add a little vinegar to increase saturation or dose two-part in addition.
The kalk meets my calcium demands, but I have to buffer alk with baking soda.