Planted Tank Obsessed
Join Date: May 2015
Location: Central Michigan
I don't believe the canister being off is the issue. I agree with nel. I've had a few 24 hour power outages during winter (I live in central Michigan) without any I'll affects on fish. The biggest factor their was water temp though. If the decreas in temp is very gradual, our fish can withstand a lot colder temps that we think. Most fish go through a hibernation cycle of sorts when they begin to cool down too much. Re-heated the tank slowly to prevent shock and everything went on without a hitch.
A lot of things can happen to our tanks during a power outage and some major things we need to consider regarding a power outage are: water circulation, water temperature, and gas exchange (oxygenated water).
Some more info like tank specs(size, substrate, water parameters, etc), fish species (type and quantity), how you cleaned the filter prior to no power, how proficient and frequent your maintenance routine is(water changes, fert dosing, co2?)
From my experience, I'd pin it down to either poor oxygenation or a quick substantial temperature swing.
Unless notoriously delicate, fish are actually a very hardy and resilient animal and when they die in our tanks it's usually because of an extreme circumstance like suffocating fish with co2 injection, large amounts of ammonia from lack of biomedia/maintenance, temperature shock during poor acclimation, a heater sticking "on", etc.
Water changes are indispensable when trying to overcome tank issues. When in doubt, do some water changes.