I had egg crate on top of my 20g and didn't like how the tank looked with it on under the metal halide, the tank is noticable brighter with it off and it didn't have a checker board reflection. A couple of months ago I put a glass lid that covers about 80% of the top just some room at the back for my HOB skimmer and filter. I have observed no difference in coral growth or colors and much less evaporation. I am also running T5HO's now to prevent over heating. I do have to clean the glass every week however to prevent build-up on the glass. So I think it is more of a maintenance issue and fish protection but a top could also trap heat if using MH IMO as evaporation is a major tank cooling factor.
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