Hey JohnNYC,
I am assuming you are on NYC water, as am I. I decided to keep livebearers in one tank (plus a quarantine/hospital tank), so I use crushed coral in the main tank. Anyways, here is my suggestion:
Get a stable, target KH/PH/GH with an amount of crushed coral. Consider the fish you are keeping, obviously. When you get it where you want, and stable......try to replicate that in the hospital tank. I do it with baking soda and Seachem Replenish (similar to Equilibrium, but with salts). In order to know how much backing soda you need to hit your target, just do a test with a given amount of baking soda in a given amount of water. (Example: see what 1 teaspoon does to a 5 gallon bucket, as far as PH/GH/KH). Then, you can accurately calculate, and get your quarantine tank the same as your main tank with the crushed coral.