I've wanted to experiment too with sealing larger nano reefs, in the five to ten gallon range. With tanks of that size, you can have quite a miniature reef. Sealing may be easier because those tanks tend to have the black plastic lip, and a solid/custom glass lid could be cut for you at any glass or window shop.
if you were to refresh the air inside continually with ain air pump and leave a small pressure vent for escape, it would reduce evaporation (not completely) and still export enough CO2 out of the system. That is, if your fish bioload isnt too heavy.
The real benefit of sealing a system comes when one includes a baffle refugium, so you don't have to let any air escape. All CO2 is processed within...its not very hard to do, just some sheets of acrylic and some macro algae and a light to hit it from behind. The quality of the seal and the water level would determine how much salt creep escaped, and in a tank that size it is very likely to run an entire month needing no top off. Id say that is pretty low maintenance! You would need to dose it regularly to keep the calcium and alkalinity up, but this is easily done with something like C-Balance in a ten gallon aquarium.
Heating may be an issue- a medium deskfan will probably need to run continually on a larger sealed system unless you have a very stable, cool room. Salt creep is a problem even with good seals, but at least that plastic lip would keep it all on the inner diameter and it will cake up around the edges--nothing a custom canopy can't hide.
Lastly, I really think to seal it to any degree you'll need to have a single glass sheet to do it right. Not sure how the store-bought lids would fare against the heat from the lights and the air from the fan. think it would collect lots of salt creep between the seams.
Good luck
brandon429
