pH range between 8.0 to 8.2 are acceptable and ideal in many folks opinion.
When the pH rises up above 8.3 or so, much of the less toxic ammonium changes over to the more toxic ammonia, especially if we keep our tanks at higher temperatures.
Obviously this can cause delicate fish and inverts some serious problems.
Higher pH typically will make it harder to keep the tankwater saturated with calcium.
Lower pH values below 7.8 or so, can lead to problems as well.
Normally, many of the nuisance algaes and cynobacteria will tend to grow better when the pH is down.
The amount of calcium and total alkalinity is also affected by the pH.
If you have extreme low pH, you may find some dissolving of your substrate, should it be of a calcium carbonate material.
If this is happening - you're in deep you know what if you have fish/inverts.
Swings during the day are fine - but a constant high or constant low would indicate something out of whack with the tankwater/tank.