We all seem to have our own formulas with regard to water. Mine is tap water with Prime added changed daily at around 15% per day. Six almost fully grown Discus, no other fish except 2 Green Cory, sharing a 30 gallon tank. This has worked well for me for the past 18 months without a problem. I have one pair spawning so I plan to move them to a 20 or or 30 gallon tank of their own and then not add any more to the 30 gallon tank. The tap water is around 7.5 ph when I test it from time to time. Something to consider if you purchased your fish from a local seller is that your fish, by the time they are introduced into your tanks, have lived in at least two different water conditions. One at the breeder, one at the wholesaler, and then your aquarium. The fish seem to adapt to these these changes without many problem.