The thing about fw is that parameters are not ubiquitous. Aftrican Rift lake Ciclids v.s S.A. Acid water Angels are pretty much as different in TDs as night and day. Most reef fish will go into the same params as far as I understand things. Fish live in different temperatures, different TDS and different pH.
So the general rule of thumb is to keep fish that work well in one's tap water. When that is too nasty to use and one must mix their own water, the whole spectrum becomes available. However, soft acidic water is much harder to keep stable than hard alkaline water.
If one is new to fw and in your situation, I would suggest using specif premixed "ingredients" made for specific types of fish. This means you need to decide what you want to keep and how much chemistry you want to master to do so.
My advice is to start with an easier route. You can always add new tanks or redo existing ones. That pretty much means a pH neutral and up. it also mean TDS from about 80-120 on the low end and the several 100s on the high end.
