pH swing - please help

[themartins]

What would cause a sudden pH change? I tested water parameters because fish were behaving oddly; not swimming just hanging out on the bottom. The pH has been stable since the beginning at 7.8; today it's now 8.2. This is a 30 gallon tank that's been fully cycled for a few weeks.

What should we do now?

Other test results:
ammonia 0
nitrite 0
nitrate 10
SG 1.026

 

[ccCapt]

It's very normal for the pH to fluctuate. It's usually lower in the AM and gets higher during the day when the lights are on. Nothing to worry about.