Besides the inevitable jumper, covers are needed to keep evaporation to a minimum. Humidity, and the subsequent mold that follows is always a problem in fishrooms, since many tanks are packed into a smaller area. Here's a link for making a cheap cover;
http
/www.gcca.net/howto/tank_covers.htm
I don't bother with the hinge deal, I just cut the diffuser panel to the size needed. A tightly sealed 29-gallon tank loses about 1/4" weekly due to evaporation; an uncovered tank loses nearly 2".
Mold is a health problem; I installed a bathroom fan & timer in my fishroom to vent out the humidity a few times daily. Condensation will eventually lead to mold.