I have a 30 gal tank with approx 20 mollies and guppies. The tank has been set up for about 1 1/2 mos. The water readings are fine except the ph stays too high. I will have the ph fine and then a day or two later I check it and it is 8.5. Why does it get so high so quickly? What is the best way to keep it stable and close to neutral? My mollies are developing rust colored spots and I am fairly certain it is because of the ph. Please help.