If there is no Ammonia, then this pH is fine. One of the things that could drive up your pH is a high total alkalinity, and water that has too high of a hardness can eventually be hazardous especially to small fish.

You should check your Total Alkalinity (Look it up!) and make sure it's about 100-180 PPM and you should make sure your hardness (not as important) is under 250 ppm

If these numbers are okay, then the pH is also okay.

Plants and animals respire, (plants at night and animals all the time) and they produce carbon dioxide which converts into carbonic acid bringing the pH down when carbonates are not there to buffer the change.

In the daytime, plants use up much of the carbon dioxide and you may see high pH.

High pH is not inherently bad for fish. (Unless there is an ammonia accumulation in the system which would be more toxic at a higher pH)

Another consideration is that you might be using a narrow range pH test kit, if you are, it shows blue at little more than 7.9

If your pH is high all the time then it may mean that there is considerable carbonate leaching from concrete grout or surfaces in the construction of the pond.

String algae loves some high pH! It's just fortunate that Plecostomus loves 'em some String Algae! (Look it up!)