Don't worry about
RO too much and pH too much. Don't go looking for work to do, that is for sure.
8.0 would be ok for most fish IF it is stable, any higher might cause a problem. People keep Discus (in healthy condition I might add) in pH of up to 8 and although this is not their water in the wild they adapt well.
But if you are really worried........
The only thing I would like to know is your
KH. If you
KH is high then you could dilute with
RO until you had a
KH that was lower, but still provided buffering capacity. In which case to get to pH 7.5 you would have to dilute approx 1
gal of tap water in 5 gals of
RO water. or 2 tap in 10
RO and so on. If you are accurate with this you shouldn't have too many hassles
HOWEVER, If something other than carbonate (ie
KH is low) is causing a high pH then you may have little buffering capacity and using
RO, while "fixing" the pH, also causes
KH to drop and your tank may be then suseptable to pH swings.
Check out what your
LFS uses, visit them every couple of weeks and see the conditions of fish. Often a
LFS will order a huge number of some fish...dependng on what they sell.... Use these as an indicator of water quality. Keep an eye on those fish over the weeks and see if they suffer as a result of exposure to
LFS water.