You have to somehow vent the lid of the tank. I was having the same problem with my Marineland Eclipse 2 aquarium. The hood of the Eclipse houses an overhead sump filter so there's no removing it. One of the T-8 light fixtures under the lid stopped working so I started putting LED lights under the hood on the acrylic support bar too close to the water. I ruined two expensive light fixtures with the condensation. Then I took a small cap from a four ounce Nalgene bottle and used it to prop open the lid less than an inch. I also elevated the light 3/4 of an inch more from the water's surface. So far the fixture is still working after about five months. I haven't had any issues so far from venting my cover.