Emergency power for fish: the minimum viable backup
The cheapest setup that keeps fish alive during a power outage. A USB air pump and a phone charger battery. Total cost under $30.
When the power goes out, the filter stops, the heater stops, and the air pump stops. Of these three, oxygen is the emergency. Temperature drops slowly (a well-insulated tank in a heated house loses maybe 1-2 degrees per hour). Ammonia builds slowly too, since the filter bacteria stay alive on wet media for days even without flow. But dissolved oxygen in a stocked tank can drop to dangerous levels within a few hours, especially in warm water, which holds less oxygen than cold.
Fish gasping at the surface is the visible sign that you are already late.
The $25 solution
A USB-rechargeable air pump and an air stone. That is the entire minimum viable backup. Plug the pump into a USB power bank (the same kind you charge your phone with) and run an air stone in the tank. The bubbles break the surface, drive gas exchange, and keep oxygen levels high enough that your fish survive.
A basic rechargeable air pump costs $10-20. A good rechargeable model runs about 20 hours continuous, or 40 hours in power-save mode (15 seconds on, 15 seconds off). Generic USB pumps run 15-25 hours. Either works. A 10,000 mAh USB power bank (about $15) extends runtime further. Between the pump's internal cell and the power bank, you are covered for 24-48 hours on a single charge.
Total cost: under $30 for equipment that sits in a drawer until you need it.
What to do during the outage
Run the air pump. That is the priority.
Stop feeding. Fish can go days without food. Feeding during an outage adds waste to water that is not being filtered, which accelerates ammonia buildup and consumes more oxygen as it decomposes. The single best thing you can do for water quality during a blackout is not feed.
If the outage is in winter and your house is cooling down, wrap the tank in blankets or towels on the sides and back. Leave the top open for gas exchange. Most tropical fish tolerate temperatures down into the mid-60s F (17-18 C) for a day or two without lasting harm. They slow down, eat less, and reduce metabolic demand. That is fine.
If the outage is in summer and the tank is warming, float a sealed bag of ice in the tank to slow the temperature rise. Warm water holds less dissolved oxygen, so keeping temperatures down also helps the oxygen situation.
If you do not have a backup air pump and the power is out right now, pour a pitcher of tank water from about 30 cm above the surface back into the tank. Repeat every 15-20 minutes. The splash drives gas exchange. It is tedious, but it works.
What about the filter?
The bacteria colony on your filter media stays alive for 24-48 hours without flow, as long as the media stays wet. Do not rinse it, do not let it dry out, do not take it apart. When power returns, the filter restarts and the colony resumes processing. You may see a brief ammonia or nitrite spike for a day or two as the bacteria catch up, especially after a long outage. Do a 25-50% water change when power returns and test daily for a week.
If the outage lasted more than 48 hours, expect a mini-cycle. Test ammonia and nitrite daily and do water changes to keep both below 0.25 parts per million until the bacteria recover.
Scaling up
For a single tank under 200 liters, the USB air pump is enough. For a fishroom or a tank over 400 liters, look at a small portable power station (500-1000 Wh) that can run a larger air pump or a small circulation pump. The solar load calculator helps size the battery if you want to add a panel for recharging during extended outages. But for most hobbyists with one or two tanks, the $25 solution handles everything short of a multi-day grid failure.
Building a $30 backup kit
The cheapest effective backup for a single fish tank:
Cordless D-cell air pump ($10-15). Available at fishing/bait shops or online. Runs on two D-cell batteries for 12-24 hours. Produces enough airflow for one air stone in a tank up to 200 liters. Buy one and keep it in a drawer near the tank with fresh batteries installed.
Air stone and tubing ($3-5). Connect to the battery pump. Drop the air stone into the tank when power fails. The bubbles provide aeration and surface agitation that maintains dissolved oxygen.
Extra D-cell batteries ($5-8). Keep 4-8 spare D batteries next to the pump. Alkaline, not rechargeable (rechargeables lose charge sitting in a drawer; alkaline hold charge for years).
Total: roughly $20-30. This kit keeps a moderately stocked tank alive for 24-48 hours on stored power alone. The most common failure mode is not having the kit at all when the power goes out at 2 AM.
When the outage outlasts your backup
For longer outages, you need either a way to recharge (a car power inverter, a small generator, or a solar panel with charge controller) or a strategy to reduce oxygen demand.
Reduce feeding. Stop feeding immediately when power goes out. Digesting food increases fish oxygen consumption. A healthy fish can go 3-5 days without food with no ill effects.
Reduce temperature if the tank is warm. Cooler water holds more dissolved oxygen. If the tank is above 26 C, float sealed bags of ice to bring the temperature down to 22-24 C. Don't drop below 20 C for tropical fish.
Manual aeration as a last resort. If the backup pump dies and you have nothing else, pour water from the tank back into itself from 30-40 cm height using a clean pitcher. The splashing entrains air into the water. You need to do this every 30-60 minutes during critical periods.
The solar load calculator can help you size a small solar backup system for permanent outage protection.