one of our customers just had a problem like this last week. I had her bring in a sample of the water she was using and the pH read 8.8 and ammonia was 1.0. After she switched water sources she no longer has any problems w/ pH and ammonia. I would test the water she is using and see if that is the problem. Change the media once a month on canisters, every 2-3 weeks on power filters.