What they said! And goldfish are pretty tolerate of more extreme pH levels. But 7 or 8 is vague and a 10x difference between them. If it fluctuates that is problem. Like they said, stability it the most important thing. It can be good to know if you have a high pH though, like 8.0+ (like me) cause some things react differently in higher pH. Most notably is that certain things are more toxic at higher pHs, like ammonia-nitrogen. Hence why keeping up on water changes is crucial in alkaline water.