So I bought a used 30 gallon fish tank, it was really dirty so I used soap to clean it. Today I took water from the 30 gallon fish tank to get it tested. Petco said everything is good except the PH. It was very high, I think because I used the soap to clean the fish tank, it left residue in the tank, the petco employee told me to do a 50% water change, I did the 50 % water change, what should I do now? How do I get rid of the soap residue and lower the ph?