Most tropical fish will do OK tap water...i'm also in the same boat as you, my GH from the tap is 26, PH 7.9 and TDS of 600. I wanted to start to keep more soft water fish like blue rams, green neons and the like. I also opted for RO, that said my RO is re mineralised. There are some advantages to using RO (other than the cost). There is obviously a better fish selection (normally more colourful), the next biggie is the water clarity, with RO the TDS remains really low, around 50-100 with rematerialised. That said, looking at the fish you keep you may be able to get away with tap water.