If you talk to even the most hardcore discuss breeders they will tell you that as long as there fish are acclimatised right you can keep perfectly healthy discuss at in tanks with high ph, gh and kh. i wouldnt worry to much unless you want to keep somethin wild that has lived in soft water. a good 50 50 mix or ro/tap should be fine for most common tropical fish. if you are getting your fish from a shop near you then there gonna have been kept in your tap water 9 times out of 10