perhaps we should look at why you have hard water first..That way you can eliminate it, if possible, without taking any action to counteract it.
What is your water hardness out of the tap, is this the reason for high figures? At least then you know you need to combat it. What is your type of substrate, your decor (type of rocks), what filter media are you currently using (hopefully not cc?), do you have any driftwood? Basically anything that is raising the levels.
I'm also assuming your fish do not like hard water?