I have mine mostly uncovered right now. Before I started keeping plants (about 6 weeks), I always kept a cover on, but I have such hard water that they are severly coated with hard water marks, and greatly cut down the amount of light getting into the tank. So I just made a fish blocker for the back of the tank, and if they jump up they will just hit the canapy and fall back in the water.
Evapoation is INSANE !!!!!!! Especially this time of the year ( winter in the northern mid-west)