The priority order
Getting this right is what turns an apparently large problem into a small one. Not everything in an aquarium matters equally when the power stops.
- Oxygen — urgent. Without water movement, gas exchange at the surface effectively stops, and dissolved oxygen falls. This is the fastest-acting threat, and it is met by the smallest load in the whole system.
- Temperature — important, but slower. A body of water has enormous thermal mass. A tank cools over hours, not minutes.
- Biological filtration — matters over hours to days. The bacteria in a filter need oxygenated water flowing through the media, and they begin to die back without it.
- Lighting — irrelevant. Fish are fine in the dark. In a planted tank, plants stop photosynthesizing, which reduces oxygen production but also reduces demand. Leave the lights off.
Most people, faced with an outage, plug in the light first because it is the thing they can see. It is the one component that should stay off.
Oxygen is the cheap emergency
A battery-operated or mains air pump running an airstone costs a handful of watts and does the single most important job.
The mechanism worth understanding: it is not the bubbles themselves that oxygenate the water — it is the surface agitation they cause. Gas exchange happens at the water surface, and a still surface exchanges very little.
Which means a couple of free measures also help:
- Lower the water level slightly so any returning filter flow splashes rather than flows smoothly.
- Stir the surface by hand periodically. Crude, free, and genuinely effective in a pinch.
- Stop feeding. Uneaten food decomposes and consumes oxygen, and fish are fine for days without eating. This is the single most useful thing you can do for free.
- Do not disturb the substrate. Releasing trapped gas into a low-oxygen tank makes things worse.
Temperature: slower than you think, and manageable
Water's thermal mass is the aquarist's ally here. A reasonably sized tank in a heated house loses temperature slowly, and most fish tolerate a gradual drop of a few degrees far better than a rapid change.
The measures, in order of value:
- Insulate the tank. Blankets or towels around the sides and over the top. Free, and it slows the loss substantially.
- Heat the room instead of the tank, if you have any heating at all. The tank follows the room.
- Run the heater in bursts from a battery. An aquarium heater is typically 50 to 300W depending on tank size — expensive to run continuously on a battery, affordable in short cycles.
- Do not add hot water directly. Rapid change is worse than gradual cooling, and tap water needs conditioning.
The load arithmetic follows the same logic as block-running a fridge: heat the mass, switch off, let it coast.
What it all draws
| Component | Typical draw | Priority |
|---|
| Air pump | 2-8 W | First — days from any battery |
|---|
| Filter, hang-on or canister | 5-30 W | Third |
|---|
| Powerhead or circulation pump | 5-25 W | Alternative to an air pump |
|---|
| Heater | 50-300 W, cycling | Second — the load that decides your battery |
|---|
| Lighting | 10-100 W | Leave it off |
|---|
| Protein skimmer, marine | 10-40 W | Useful on a reef tank; not urgent |
|---|
Working estimates from typical aquarium equipment ratings rather than manufacturer measurements — the ranges are wide because tank size drives everything. Your own equipment's labels are the figures to use.
A pump and a filter together is under 40W, which any battery covers for days. The heater is the only component that changes the size of the purchase.
Marine and reef tanks are harder
Worth being honest that this page is written mainly for freshwater.
A reef tank has more demanding requirements: higher circulation, tighter temperature tolerance, and livestock that is both less forgiving and considerably more valuable. Corals in particular tolerate less variation than most fish.
Practically that means:
- Circulation matters more, and a battery-backed powerhead is worth having rather than just an air pump.
- Temperature tolerance is narrower, so insulation and burst heating matter more.
- The value at risk is higher, which changes the equipment budget calculation entirely.
A reef keeper should size for the real load rather than the minimum, and the kilowatt-hour tier is the sensible starting point rather than the ceiling.
The plan
- Keep an air pump and airstone for the tank, permanently, with spare tubing. It is the cheapest insurance in the hobby.
- Put it on a UPS, which covers every short outage automatically without you being home.
- Have a battery for longer events, sized on the heater rather than the pump.
- Insulate first, heat second. Blankets are free and they reduce how much heating you need to buy.
- Stop feeding as soon as the power goes.
- Leave the lights off throughout.
Done that way, an aquarium is one of the smaller backup loads in a house rather than one of the larger ones — and the same battery that covers it covers the fridge, which is the purchase that actually makes sense. Sizing it properly works both together.
