I am currently using my old heater from my 37 gal tank for my QT since I have it already but it's a 200w and my QT is a 10gal. Is that dangerous? Should I go buy a 50w?
The only potential problem is if it 'fails' in the ON position, it will cook the fish alive very quickly. It's rare, but it has happened to heaters before. A smaller heater would give you time to realize it climbing without killing the fish.