I had an unfortunate electrical blackout due to an icestorm that lasted 5 nightss 6 days temp inside the house dropped to below -7Celcius. It was a gradual drop to -7 on the fourth night and below that on the 5th night Never below freezing
No Co2, No Lights, No filtration in 9 of my tanks some with heavily stocked fish / some died in tank with plants. I stopped feeding fish and fertilizing when filters went out and tried water changes and removed dead fish as much as I could without light but eventually hot water tank lost its hot water and I added little fresh water for aeration and cleaner water. It was actually quite interesting to see when the lights came back on that some of the plant roots like JAva Fern had wrapped themselves around / or the fish got caught in then died - It loooked like the plants were feeding on the fish carcass.
But once power came back on Plants flourished like never before. Mind you some were close to death ie Donoi and Syng Manaus Ludwigia Pantannal in my 30 Gallon Long Tanks seemed to be hardest hit. The Bigger Taller and larger tanks seemed to handle the stress better.
Sword TAil, Mollies, Celestial Pearl Danios and Burmese Loaches survived. Very strong Fish
Im almost tempted to try this again in the future on purpose this time JUst to get those results, but only in tanks that I know the fish can handle it.