Aquarium cover/lid.

Evaporation equals cooling. With our lighting and pumps we generate quite a bit of heat that often requires a way to dispose of it. A chiller is one way but can be expensive to buy and to operate. Fans and/or lots of surface agitation give you much the same end result for lots less money. I have a 1/4 HP chiller but it has not run since April 2008 entirely due to fans and evaporation. Yes I evaporate about 3-4 gallons of water daily but thats where an auto topoff device comes in handy.
I am in Phoenix and my home has a setback thermostat on the air conditioning when we are not at home during the day, so it gets pretty warm in the house. With the fans and no glass tops my tank stays 79-81 year round, same as what the chiller was set at and it cycled at least 4 times a day in the summer.

 

I had a completely closed top for say 16 yrs. Always had surface agitation, it isn't like the glass is laying on top of the water. Aeration can come from a variety of sources so that isn't an issue. Evaporation does not equate to cooling it is a result of uneven temps (water to air). Keep the glass clean and only minuscule losses of light occur.