Hmmm...have not tested the ph of tap water before. For tank water, did you test it when ur lights were on? When lights on, PH increases, lights off, ph decreases. To increase ph, you can use buffer solutions e.g. seachem buffer or just dose carbonate. Try to maintain ph at ard 8.1 to 8.3 when lights are on.
Another thing to do is to check your dkh or carbonate hardness.