The biggest thing with Ph is keeping it stable. If it's generally around 7.8 and then the next time you test it it's at 6.5, there's a problem and you're stressing your fish. Also, if you have pretty hard water and very low ph (or the opposite), check and see what might be going is going on -- high nitrates, co2, low oxygen, overstocking... just look it could be nothing, but it's pretty unusual.