Well, today I found a simple solution if you have heaters in your tank, and the power goes out for a longer period of time, not just a few minutes.
Usually dont have heaters in my tanks in the house, except for one at the moment, where there is a dropsied ranchu in there, and a lowering of the water temperature could prove fatal for him. Power outage was around 2 hours, so the temperature would have dropped. Well, I took a 1 liter soda bottle, filled it up with warm water, a few degrees hotter than the heater is set, and put it in the ranchus hospital tank. The temperature remained constantly at the 85 degrees I have set the heater on before the power went out. Simple, cheap, and effective! Can safe a sick fishies life!!