The pond scum that forms on the top of eutrophic ponds is a non-issue generally on a sunny day, because light penetration is allowing for photosynthesis. The only time it is a significant issue is during the wee hours of the morning when it's been calm all night and plants are respiring. This can be the summer kiss 'o' death for your fish. Early morning summer fish kills are devastating when they happen.
No wind, no photosynthesis (respiration), lots of B.O.D. and pond scum can wipe out all of your hard work in about 30 minutes.
So, in answer to the question, you could theoretically run the circulator from 3 a.m. to 7 a.m. all summer and early fall to help your chances of keeping your fish population healthy.
