I used fish tank heaters for years with no issues until one of them came apart in the FV. I could hear a faint buzzing in my room and after much snooping around, when I switched the heater off, the buzzing stopped. When I opened the FV, the glass tube of the heater had separated from the rest of it. Worryingly, it was still electrically live sitting in the brew. The brew had to be discarded and that kinda put me off fish tank heaters. On the bright side, at least I did not electrocute a tank full of fish!