Betta Fish: heater help?

i bought this heater at petco a few days ago without checking reviews 🫠 as soon as i put it in i noticed it floats around and doesn’t stay still against my glass tank. it’s a 6 gallon cube. honestly it was a bit pricy so does anyone have any tips on how to make it work?? also i checked temperature and it was at 83°, but i’ve heard that it’s better for a betta to be at 78-80°… it’s not a preset heater, so any tips on how to maintain a nice temperature? or if anyone knows another affordable heater i can replace it with? (btw fish isn’t in tank yet i wanna get everything down before buying one)