Just wanted to share an update! Have been doing small daily water changes, installed my new aquaclear filters (with sponges from old filters), and added a small amount of crushed coral to my tank. My pH has come up to roughly 7.0. I found out that my city water supply is soft and has no buffering capacity after talking to other fish keepers in my area and my LFS. I haven't had any problems with my 120gal because it has a sand substrate that has crushed coral in it. I added small amounts of the crushed coral to my 55g that had the pH issues and to my two 10g tanks where the pH was also low. Everything is good so far and my fish seem to be doing great now. I will continue monitoring the pH and up my water changes to 2 times per week to keep the nitrates and waste from building up. Hopefully the pH will remain stable. Thanks again to everyone for their help!! I truly appreciate it.
