If you mean using hard water on your tanks?
I wouldn't advice you to use it, actually I'd say is the best to avoid it.
Hard water is not even good for human usage to be honest, I have to drink this sh*t everyday, since our tap water is hard water, it ain't good.
Well, my hard water also contains a bit of poison on it as well (LOL!) but not as much for human health problems, but would totally be bad for a fish for an example.
Try to find your tap water quality report, usually you can find at the government/state page.