I do that also, no cover, maybe glass cover when its cold in the winter so the heater does run constantly. The tanks need evaporation to cool in the summer, never going to buy a chiller. I read a good article on it. Suspended lights, alternating pumps, tank is a few degrees above room temperature now. I had to read about it allot, T8 lights were not the major factor in my case, the 402 power heads pushed the temperature over 86 degrees. I was not happy, so I thought my way through it, put the spray bars on a Natural Wave maker. By accident I designed surges and turbulence, lowered temperature, and got Coralline Algae to grow very rapidly. Never a bad idea to do the research again, things change.