Originally Posted by kif3
but doesnt having a different ph mean he can get infections more easily?? :s
No it does not. Where did you read that?
Ph is just the level of acidity in the water, it doesn't really have anything to do with infections or sickness. Bettas thrive in many different phs. Some members on here, like yourself, have ph in the 6-7 range, whereas some members, like myself, have higher phs in the 7-8 range. Both are fine. It is just a matter of adaptation like the others said.
A water change of around 25% should be fine, just make sure the temperature is pretty even. To be safer, you could add back the fresh water a bit more slowly. Like, instead of just taking the 25% out, and immediately refilling the tank, you could refill like 5% every 20 minutes until it is full again.