Before the cold comes
- Size the heater for the room’s coldest night, not the average day. The colder the room, the more watts the same tank needs; makers’ charts give the size for the gap between room and water. Heater size for your room
- Check the heater works: its light should come on and go off, and a thermometer, not the heater’s dial, should show the temperature you set.
- Keep a spare heater, and in a big tank use two smaller heaters instead of one large one: if one fails off, the other keeps the water from crashing; if one sticks on, a smaller heater warms the water more slowly, which gives you time to catch it on the thermometer.
- Move the tank away from cold drafts: an outside door, a cold window or a floor vent.
A lid and a cover
Most of a tank’s heat leaves through the open water surface, as evaporation. A lid that covers the top keeps it in and lets the heater work less. On the back and sides, a sheet of polystyrene or a fitted background cuts the loss through the glass in a very cold room.
When the power goes out
- Keep the lid on and wrap the tank in blankets, towels or a sleeping bag, leaving the front open a little so you can see the fish.
- Keep the water moving with a battery air pump: without the filter, oxygen falls and waste builds.
- Stop feeding until the power is back; food without filtration turns into ammonia.
- If the room gets cold and the outage goes on, float sealed bottles of warm (not hot) water in the tank and replace them as they cool. Do not pour hot water in.
- Check the temperature every few hours. A slow drop of a few degrees is far easier on the fish than a fast one.
When the power comes back
Let the heater bring the water back up on its own; it warms slowly, which is what the fish need. If the filter was off for more than a few hours, rinse its media in tank water before you restart it, because bacteria in still media die off and it can release a slug of dirty water. Test ammonia over the next few days. Restarting the filter · Ammonia