Agree with Kristie; you run a chance of shattering glass with UTH. There are various underwater heaters made for small 1 gallon Betta set-ups to 5 gallon tanks. Estimate the volume of water you have in tank and get an appropriate sized one.

Usually they are set-up from factory to around 78F. Do not run them above water; they have an overheat thermal protector and will shut down permanently if sensor is triggered. Good luck !