I believe most would suggest you first check to make sure you dont have
anything which will naturally raise your pH such as limestone rock, concrete
blocks (that's the only two I can remember). If not, a stable pH even
though on the high side will be fine for your fish. It is the significant
pH swings which are bad for the fish. I would think your swing is very
common. Sorry I was going to say no problem but there are also some plants
which wont thrive as well (??) and ammonia (if it developes) is much more
toxic at higher pH.
HTH some
Bill Brister - Austin, Texas