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?