Hey Al, is your tank glass or acrylic?
I always believe in keeping the top open for cooling and gas exchange. However if your tank is acrylic and you are using single ended bulbs, I have been told that you should consider some type of UV shielding between the bulbs and top of the acrylic tank. Apparently UV can compromise the integrity of the material. I'm not sure why this is not the case with dbl ended bulbs.
Also, if you have any jumpers in your tank, you may consider putting some eggcrate on top of it. This would allow you to keep the fish in, and maintain cooling/gas exchange.
HTH!
Rob
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