A damp towel on top or I made a cheap humidifier when I was using a glass tank. I think I got out for under $15

http://m.youtube.com/watch?v=d6Cgx4x0F_g

Mine was similar just with an airstone at the bottom. I had it on a timer and covered a good portion of the screen to with plastic wrap. Worked pretty good. I didn't like a damp towel on top, it worked but if I let it dry out my gf's cat would post on top of the cage and it make me nervous


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