Hi. I agree with Rob w. The best pH is the correct pH for the fish. However that is not always practical, so my answer is the best pH level is generally the pH of your source water. As long as you pH is somewhere between say 6.5 and 8.5 then i wouldn't worry about it too much. Most fish can adapt, or are adapted to a less than optimal pH, and you can cause much more trouble by trying to adjust it that it's not worth the expense or bother. A consistent pH is much more desireable than an ever changing pH due to adding chemicals.