I wouldn't mess with the pH. Is there a particular reason why you would want to? If you are buying local fish, they are generally acclimmated to the pH of the local water. Mine is very high (almost 9) and many fish have thrived at this pH. The ONLY reason I have added RO water to lower my pH is because I am breeding fish in which pH and water temp determine the sex (and amount) of fry. IMO, don't even bother: I tried it (pH down) about 1 1/2 years ago, and it worked for about an hour or two before it shot back up again. Something about the buffering capacity. HTH