The pump should be on a timer and circulate regularly. The circulation moves the heat from the bottom to the top and from one side of the tank to another.

I’ve witnessed cases where the bottom of the tank was over 100 degrees but the top of the water was significantly coder.

I have a timer with 2 outlets. This is convenient because it allows me to put the UV power into one outlet and the actual pump in the other outlet.

GE makes them, so does UltraPro and others.