tank cover glass or plexi?

I just purchased an established reef tank, that has glass lids on it. I was talking to a friend of mine that has a fish only tank and he said that I deeded plexi glass covers to allow a more natural light spectrum thru. I remember seeing somthing on this years ago when I had my first tank, but I thought it was the other way around. Wich is right glass or plexi?

tyohnk,

I think most here will tell you to not use any covers at all on your reef tank. They generally tend to say: - It prohibits good gas exchange. *Huge B.S. near as I can find calculations confirm.* - It holds heat in. *Get a chiller, and/or have less need for intense light by keeping cleaner water, and/or run halides less time per day.* - It prohibits light passage. *have less need for intense light by keeping cleaner water, and/or run halides less time per day, also tilt the lids so droplets tend to flow back to the tank instead of hang down, make sure to keep the tops clean on the outside and the salt creep clear on the inside.*

I'm sure there may be one or more I missed, but for what it's worth I use the 'Solar' glass Oceanic provided and whatever comes on third party tops you might find in catalogs. I have five tanks so I have a mix of tops. Some glass has natural or added tint which good acrylic doesn't but I haven't found it to not matter much either way but that's my opinion not fact. Try running for a month with the tops and accumulate the dust that develops on them. Then ask yourself if that is something you would *feed* your tank on a regular basis. If it is take the tops off, if not then leave them on but keep them clean. You will also be well served to have a filter on your skimmer's air intake if it is of that type.
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Be careful if you have fish that will jump from the water. I've had good success with flourescent light diffusers to keep fish in. They are the plastic grids you see on flourescent lights in offices. You can cut them down to size easily. This allows for gas exchange and heat dissipation. You can get them at any home depot in the lighting section.