I did a number of searches, but for some reason none of my searches went back past June 2001. The thread I remember was before that. Sorry, but I don't have the web site. For what it's worth, I cover my tank with 3/16" plexiglass based on research I did a few years ago which showed that plexiglass, unlike glass, blocks out most of the worst UV and lets most of the rest through. I understand that is why museums use plexiglass to encase exhibits. Plus it's dirt cheap and is easy to cut. My VHOs are only about one inch above the plexiglass, so going without a cover is out of the question. I run a small bubbler in my sump to provide additional oxygenation and carbon dioxide gas exchange. I have never had a problem with oxygen levels, but I did have a carbon dioxide problem before I put in the bubbler. No problems since. I rinse the covers weekly, but never scrub them. Also, I have the corners cut out of the covers above the overflow so I can get a grip on them to remove them and so some air gets through. Turning the lights off does not seem to lower the temperature because my chiller operates at about the same intervals day and night. However, I did notice that removing the hood entirely does keep the temperature down. So the hood and covers do seem to act as insulators. In the future, I would consider going with UV-shielded pendants hanging high enough to keep the tank top completely open, which I think would look really cool, as long as there was no problem with jumpers. For now, VHOs with covers work well for me. Good luck.
