I saw on another post that someones recommending this. I hope that noone thinks this is ok.
i would NEVER set any size aquarium on top of a heating pad. a gallon of water weighs approx. 8.35 pounds.
multiply that to how many gallons ur tank is. thats ALOT of weoght to be putting on a heating pad that is NOT meant to be used that way. you could cause a short in the heating pad, resulting in a fire or electrical shock. the elements inside a heating pad are not designed to take that sort of force on the thin elements that run thru it.
also, most newer heating pads have automatic shut offs when they run for a certain time. and the older ones that dont have this feature could be extremely dangerous under a tank for that long.
and what happens to the pad when u do water changes and it gets wet?
this has disaster written all over it.
please dont use heating pads under your tanks. it could have dire consequences.