I think I know the answer, but I need some experienced advice. I just moved my Betta from a 1 gal jar (shame on me, I know, I was a newbie not too long ago!) into a 5 gallon tank. Put a heater in it, on low to slowly acclimate him. HOwever the heat wouldn't rise since there was not top on the tank. I made a makeshift tank top of stiff cardboard, covered in foil, which very nicely warmed up the tank for him, but then I realized that the foil, which is a little moist, may be dangerous. Any input is appreciated. What can I use to keep tank covered if this isn't a good idea?