Alot of tropical fish are pretty hearty and will survive at temps around 60°+ especially if the temperature drops slowly due to a lack of heating. It's not optimal and some fish will not tolerate it, but you'd be surprised. I haven't had a power outage in a couple years but I've had my share of heater failures in that time period.... Explosions, thermostats sticking on, sticking off. my community tank dipped down to the low 60's (room temp) after a fluval heater failed. Nobody died, some fish actuay laid eggs, I guess it was a spaening cue. I had corydoras, zebra danios, angelfish, bolivian rams, and some other miscellaneous community fish. They all lived.
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