Some people were trashing the Deep Bule heaters for a while, but I've had one in my girlfriend's tank for the past year. It does get unplugged during the warmer months. The Heat Stik is what we have been running. It fits nicely in the filter and keeps a 7 gallon bow at a stable temperature. And by keeping it in the filter there are no worries about uneven heating, or having to see the heater in the tank. You can't adjust the temperature, but I believe the set point is between 78-80*. I think it always held 78* for us.
I believe one is for tanks up to 5 gallons and there is one for tanks up to 12 gallons. You should be able to get them for under $15. Don't get the one for Beta tanks. That one is undersized for most applications and is only rated to keep the temp 4* above ambient, which means it isn't calibrated to maintain a specific temperature and will swing all over the place and lead to the issues you are having now. The are probably designed for tanks smaller then a gallon.
Just make sure you unplug it if you are doing a water change and shutting down the filter (if you put it there). It is a glass heater, so if you leave it plugged in and it gets exposed to the air, it will shatter when it gets re-submerged.