Fishfreak, may I ask why you don't use a glass cover on your tank?
I'm not FF, but I may be able to answer your question. At the least, I can tell you why I don't use a lid either.
Lids do a couple of things to a reef tank that is undesirable. First they trap heat. Most reef tanks have extraordinary lighting used to keep corals. With all these bulbs burning, heat could easily trap underneath the lid and cause the water to heat too much. Without a lid, the heat escapes into the atmosphere causing a small impact on the temperature of the water. Some larger tanks with larger lights still have temperature increases without lids, however, the increases are smaller had there been a lid on the tank to begin with.
The second reason quite a few people run "lid-less" tanks is because it helps in gas exchange. Without a lid, the tank can vent off CO2
that builds up in the system. High levels of CO2
can contribute to a lower than wanted pH. O2
can also enter the water easier with without a lid acting as a barrier.