Keeping a Tank Warm (and Breathing) During a Power Outage

A power outage is scary for fish keepers. The filter stops, the heater goes cold and the room starts cooling down. The good news is that a healthy tank can survive much longer than most people think, if you know what to do. Winter storms and long outages are coming, so now is the time to plan.

The two real threats

1. Oxygen. With the filter off, the water stops moving, and less oxygen gets in at the surface. In a heavily stocked tank, this is usually the bigger danger, and it shows up first.
2. Temperature. A big body of water loses heat slowly, but a cold house will pull a tropical tank down over hours. Fast swings are worse than slow ones.

There's a third, slower problem: the beneficial bacteria in a filter that sits without flow for many hours start to die off. When the power returns, a dirty, stagnant filter can release a slug of bad water. We'll get to that.

In the first hour

• Don't feed. Digesting food uses oxygen and produces waste. Fish can go days without food.
• Cover the tank with the lid and lay a blanket, towel, or a sheet of foam board over the top and around the sides. Most heat leaves through the surface and glass.
• Add oxygen. A battery-powered air pump with an air stone is the single best thing you can own for this. If you don't have one, gently scoop water out with a clean cup and pour it back in from a few inches up every 15 to 30 minutes. It adds oxygen at the surface.
• Lower the lights and keep things calm. Stressed fish use more oxygen.

Keeping it warm without power

• Insulate. Blankets, towels, sleeping bags, or foam board around the glass make a big difference.
• Float sealed bottles of warm water in the tank, swapping them as they cool. Use clean, sealed bottles so nothing leaks. Heat water on a gas stove or camp stove used safely and ventilated.
• Warm the room, not the tank, if you have a safe heat source rated for indoor use. Never use a fuel-burning heater that isn't made for indoor use.
• Hand warmers in a sealed bag, taped to the outside of the glass, can help a small tank.

Most tropical fish tolerate a slow drop into the high 60s for a while. A slow drop is much safer than trying to heat it fast. Cold-water fish like White Cloud minnows and hillstream loaches barely notice.

If you have backup power

A small portable power station can run a tank for a long time if you're smart about it. Run the air pump first; it uses very little power. Run the filter next. Heaters use a lot of power, so run them in short bursts only if the water is getting cold. Our sister site has a guide to power stations and generators for the whole house.

When the power comes back

• Clean the filter media before restarting if the power was out for more than a few hours, especially in a canister filter. Rinse it in tank water, not tap water, so you don't kill the good bacteria. A stagnant filter can dump a burst of dirty water into the tank when it restarts.
• Check the heater turned back on and the temperature climbs slowly.
• Test ammonia and nitrite for the next few days, and be ready for a water change. See our guide on water changes.
• Feed lightly for a day or two.

Put an outage kit together now

• Battery-powered air pump, spare batteries, airline tubing and an air stone
• A blanket or foam board cut to fit around the tank
• A few clean plastic bottles for warm water
• Water conditioner and a test kit
• A portable power station if you keep valuable fish

Ten minutes of prep before winter can be the difference between a scary night and a dead tank.

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