If none of the top is covered you can cover about 3/4 to 2/3 of the top with saran wrap or tinfoil. If you have a dome light and not a compact one place the dome down on the foil/wrap and cut a hole about an inch bigger then the dome so you have ventilation from that part once the dome is in place. Or you could go out and buy a fogger system but covering the top should help. I have a pacman in 10 gallon and I have 3/4 of the top covered and its been pretty dry around here since its got colder and I haven't had any problems with humidity.