From what I have seen, water chemistry for rainbowfish is all over the charts. Apparently, the origins of the fish vary widely and the PH, etc. accordingly. I'd research the many rainbowfish varieties and go with those that fit you water the best. My water is softer than yours and my PH is over 8. PH is often altered by water utilities to offer protection for pipes and appliances. Trying to change your water to match fish is very difficult to pull off consistently.