You can use a regualr ole heating pad. That's what I have my guys on right now until I can get my bigger tank built.
Mine are in the 4 x 4 x 7 beanie boxes. Their water stays around 80-82 degrees.
I did have to try it first, of course, on a wet (can't say dry you know lol) run before putting the fish on them.
I put the pad on low first, then sat the tanks on the pad. Waited an hour and checked the temp...if it was to low I felt around the pad for a warmer spot then where the tank had been sitting and moved the tank there, then waited for another hour and re-checked the temp.
If you can't get the water to the right temp after a few trys on Low..move it to Medium and wait another hour, ect.
After you reach the temp you want you'll wanna wait 2 hours and check it again to make sure the water stays that temp and doesn't slowly rise. Then you can add your fish, but make sure to check it hourly, just to make sure it doesn't raise unexpectedly on you, for the rest of the day (for atleast a 6 hour period).
Mine have been on heating pads (3 beanie boxes to a pad) for several weeks now..with no temp flucuations, other then between 80-82 degrees F.
I took a sharpie marker and outlined where the boxes sat once I found a good place for each box...so I didn't lose their spot during water changes (saves me a couple hours a day of searching for my spot again lol)
Also, you'll wanna make sure to keep a lid on the tank...as cooler temps outside and warm water inside...can give your fish pneumonia...when he comes up to get a gulp of air. If you see condensation on the inside of the tank, that's a good sign that the air temp on the surface of the water is safe enough for him to come get a gulp.
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Good Luck
Edit: You'll want to make sure you use a MOIST heating pad..they are water proof. If it billows (puffs up) because of it being sealed, you can take a safety pin and poke one hole in it..that will let the trapped air out, but won't allow water to get in..unless you drop it in the bath tub or something lol.