Newb question. Why can't you have glass lids on tank to prevent evaporation? Something about O2 surface transfer. But doesn't the skimmer O2 take place of that with all the air bubbles?
I've run glass tops for like 4 months now on my reef. I went from topping off a gallon a day to a gallon every 5 days.. My tank stays below 80 and is totally fine. I do have low pH though but its only seeming to slow down the coral growth. Which Im ok with at the moment. But I actually have 3 separate glass pieces covering the top from home depot, cause Im cheap.
I've got a 30g setup that started as FOWLR with a glass top that I've added a few corals to. The algae certainly didn't have any trouble getting light (thank god for vodka dosing) and the coral seems to be doing OK. On larger or deep setups or non-LED setups it's probably more troublesome, but glass is probably a fine option for smaller setups. Coral selection might be limited to those that don't need the brightest of lights.
I run glass tops on my 30 cube, but it has a chinese led and I clean the glass every now and then. I haven't seen any slow-down in growth, and the evap problem on this tank was substantial before the tops. There is a 2 inch gap toward the rear where the tops don't cover (as this is where the skimmer hangs on) so I do still get good air flow.
My 180 had a glass devider / support at the top running front to back in the middle. If I didnt clean the thing at least every other day the film from the saltcreep would start to cover the glass. You could totally tell the center of the tank was darker than the rest of the tank. I actually tried to run a 400 watt MH in the middle to help with this and it didnt.