Everything that was suggested applies. When I was using tanks as enclosures for my animals I had "versa tops" for all of them. Versa tops are folding glass lids that come with a plastic attachment for one side of the glass. They insulate extremely well and help with both heat and humidity. One top for a 40 gallon may be pricey but it's a investment for the comfort and health of your animals. They are sold at Petsmart, Petland, and Petco. Worth consideration and be sure to do the price check at all those locations.