JimM

They will definitely help with evaporation but it really depends on what kind of lighting you are running. If say you are running LED's then i wouldn't worry about the increased heat but if (like me for now) you are running intense halides, then you may end up having a heat issue. You may run into heat problems with T5's but that really depends on your setup.

Tom

I found thur the years that the constant cleaning of the glass covers out way any benefit, just my opinion. I have always used a canopy with a fan exhaust and a intake filter on opposite side to keep humidy form being a issue. Im not a fan of the open tank though because leaves to much of a unknown for contaminates to enter the tank like dust bringing in phosphates or in my case kids putting cookies and stuff in the tank.

I keep covers for only one reason: to splash proof my lights. Don't have MH so I keep my lights low. If a fish were to splash on my heated bulbs, the temp diff could brake/shatter the bulb glass.

I haven't had a glass cover on a tank in 13 of my 14 years in the hobby. They do trap heat, and limit oxygen exchange. Some light fixtures have a splash guard, protecting the bulbs. My Lumenbrights have glass shields, and they are way up high.

Of course, if you have jumping fish, you better have some type of cover to limit any losses. Just watch your tank temperature closely, especially as we approach summer.

Thanks all... really appreciate the insights!