I'm about 15 min west of Newark--a little south of you. I think mesh gets colder than solid...if you think about it. There's no barrier to heat rising, no greenhouse effect. Plus the mesh allows evaporation, which also keeps the water cooler.

I would say, if the water is pretty clear and 50 or below, don't bother adding anything. In fact, unless you have a reason to, I prefer, myself, to leave it alone. It's going to get colder--this absurd heat wave cannot last.