Open top for gas exchange. I'm not one to fancy many fish, so its very easy to avoid the well known jumpers like firefish or wrasses. Though suppose I were interested in these fish, I would still maintain my open top. I would certainly engineer it differently to avoid fish breaching onto the floor, perhaps by encompassing the top in a wooden canopy, with very good ventilation your fish would be unable to exit through, or by using a high albedo egg crate over the top as just a junky lid.
+1 I've been open top for years and never lost a fish--just lucky I guess. I agree that gas exchange is the best reason to leave it open, not to mention the fact that it is much easier to maintain the tank without a top on it( canopies excluded).
Nope, not fun whacking your elbows on light fixtures, I've knocked one right into the water once >_<
I use glass tops under my t5's to keep salt off my light fixture and to keep the temp up in my tank.
The glass tops and shorter than the tank is front to back, and I use two otb filters to exchange gas and regulate temperature.
I've lost a number of jumpers along the way, and check regularly for escapee snails. The snails tell me when I have voltage leaking. Most recently it was a bad pump on a skimmer.