____________________________________________________"you can only add one part of it to your top off water. or, you could setup two different auto top off tanks, one with part 1, and the other with part 2, connected to two powerheads, and going to two different parts of the tank. that might work." This will only work if all your top off water is added to the tank BEFORE you add the calcium supplement. You need to add equal amounts of each, with the buffer first.