What should be done for aquarium fish during a power outage?
Keep the water oxygenated, limit stress and waste, and prevent sudden temperature changes. A battery-powered air pump is the safest immediate backup; if one is unavailable, manually disturb the water surface until power returns.
Immediate priorities
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Create surface movement. Run a battery air pump and air stone if available. Otherwise, use a clean cup to scoop aquarium water and pour it gently back in from a short height, repeating regularly. This helps gas exchange at the surface. Dissolved oxygen is affected by temperature, biological activity, and water movement; low oxygen can quickly threaten fish, especially in crowded or heavily stocked aquariums. See the USGS overview of dissolved oxygen and water and EPA information on dissolved oxygen.
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Reduce oxygen use and waste. Turn off aquarium lights and do not feed fish during a short outage. Feeding creates additional waste and increases oxygen demand. Keep the lid in place to reduce heat loss, but do not seal the aquarium airtight.
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Protect temperature gradually. Keep the room stable and wrap the sides of the aquarium with blankets or towels, leaving the top and equipment area accessible. Do not pour hot or cold water into the tank, place it on a radiator, or use improvised heater hacks; rapid temperature changes can cause additional stress. Species, aquarium size, room temperature, and stocking level determine how quickly temperature becomes dangerous.
If the outage continues
Check fish behavior and, if available, test temperature, ammonia, and nitrite. A filter that has stopped should not be restarted if its media has dried out or smells strongly foul; rinse the media in removed aquarium water before restarting rather than using untreated tap water. Follow the equipment manufacturer’s instructions where available. If water quality deteriorates, a properly prepared partial water change may be needed, with replacement water matched as closely as possible for temperature and treated for chlorine or chloramine.
When electricity returns, restart equipment and confirm that the filter, heater, and air pump are actually operating. Avoid immediately feeding a large amount. For a prolonged outage, seek advice from an aquatic veterinarian or qualified aquarium professional, especially if fish are gasping at the surface, lying on their sides, or dying. Never operate a fuel-burning generator, grill, or engine indoors or near windows.