Power outages: what to protect, in what order

The failure order

When circulation stops, gas exchange is what degrades first. The surface stops being renewed, oxygen consumption continues, and dissolved oxygen falls — faster in a warm, heavily stocked, or heavily planted system than in a cool, lightly stocked one. This is why oxygen, not filtration, is the first thing to address.

Temperature moves next, and which direction depends on the room. A large volume of water changes temperature slowly, which is an advantage, but an aquarium in a cold room or a hot one will follow it eventually. The problem is rarely the absolute value in the first hours; it is the rate of change and where it ends up.

The biofilter is more durable than most people expect but not indefinitely so. Nitrifying bacteria are aerobic, and they live largely on media inside filters. Media sitting in a stopped canister or sump with no flow and no oxygen begins to die, and decomposing media releases waste rather than processing it. The bacteria on rock, substrate, and other surfaces in the aquarium itself are in better shape because the water around them is still connected to the surface.

Anything relying on a pump also stops: protein skimmers, dosing, chillers, automatic top-off. A stopped automatic top-off is usually harmless; a stopped chiller in a warm room is not.

What to do while the power is out

Address gas exchange first. A battery-operated air pump is the cheapest effective tool. Without one, moving water by hand with a clean cup or pitcher, poured back so it breaks the surface, does real work. Do it periodically rather than continuously; a small amount of surface renewal repeated is more useful than exhausting yourself in the first hour.

Slow temperature change rather than trying to control it. Closing the cover, wrapping the aquarium in blankets or towels, and closing curtains and doors all reduce the rate of loss or gain. Adding heat to an aquarium safely without power is difficult and easy to do badly; reducing the rate of change is usually the better use of effort. Do not add hot water directly to a stocked aquarium.

Stop feeding. Uneaten food and additional waste production are exactly what an aquarium with no filtration and falling oxygen cannot absorb. Fish in good condition tolerate a period without food far better than they tolerate a water-quality failure.

Keep the lights off. Lighting adds heat and, in a planted or reef system, drives a photosynthesis-respiration cycle you cannot support. Darkness also reduces stress on the animals.

Protect the biofilter if the outage is extended. Media sitting in a stopped canister or sump is the most vulnerable. Moving it into the aquarium itself, where the water is at least connected to the surface being agitated, preserves more of it than leaving it sealed in stagnant water.

Restarting safely

Do not simply switch everything back on and walk away. Before restoring power, consider what is sitting inside stopped equipment: a canister or sump that has been still for an extended period may hold water that has gone anaerobic, and pumping that into the aquarium at once delivers a slug of waste and an oxygen demand to animals already stressed.

If equipment has been stopped long enough for that to be a concern, inspect and, if the water in it is foul, discard and rinse the media in aquarium water rather than returning it as is. Heaters deserve particular care: a heater that was left switched on and became exposed as water evaporated or was moved can be damaged, and thermal shock can crack glass. Check that heaters are fully submerged and, where the manufacturer instructions call for it, allow them to equalize before energizing.

Restart in stages and watch. Bring circulation and aeration back first, then heating, then the rest. Then test. Ammonia and nitrite are the readings that tell you whether the biofilter survived, and they may take a day or more to reveal the answer. Test more often than usual for the following days and feed conservatively while the system re-establishes.

Preparing before it happens

The preparation that pays is unglamorous: a battery-operated air pump with fresh batteries, a way to move water by hand, an accurate thermometer that does not need power, and blankets. For systems where the stakes are higher, an uninterruptible power supply sized to run circulation and aeration, or a generator with the electrical work done properly, extends the window considerably.

Reduce single points of failure while the power is on. Independent circulation, a second heater rather than one oversized one, and equipment on more than one circuit all mean a single failure degrades the system instead of stopping it. Include equipment inspection in the routine described in the maintenance guide, because most equipment failures announce themselves before they finish.

Sources and further reading

Each source is listed with the part of this guide it supports. Source scope matters: a taxonomy record, product label, or general husbandry page does not automatically support every care claim.

• University of Florida IFAS: Dissolved Oxygen for Fish Production
  Why oxygen depletes first when circulation stops, and how aeration restores gas exchange at the water surface.
• Merck Veterinary Manual: Management of Aquarium Fish
  Biofilter dependence on flow and oxygen, and the water-quality consequences of an interrupted system.
• Merck Veterinary Manual: Essential Maintenance
  Recurring equipment checks and the observation tasks that detect a failure before it becomes an emergency.