I bought a cheap desk lamp at wallyworld, it's a regular bulb, not LED. I set it up NEXT to the glass, not over the tank. You'll have to leave it on all night but the shade and bulb base gets really hot and since it's up against the glass it heats the tank. I made it touch the glass.
I used this with a hospital tank in my bathroom, it's a 2 gallon glass cannister "tank". I wouldn't use this with anything bigger or anything plastic/acrylic. It kept the tank at 76/78 but it's also a pretty warm room.
I will take a photo and post later today.
If you have some money I would get these for anything under 5 gallons:
also covering the tanks with saran wrap helps keep heat in.