I have kept fish for most of my adult life, often in houses where the hallway never saw eighteen degrees and the tanks sat there like little heated pools, quietly defying the building around them. At some point I started metering them, because "the tank barely uses anything" and "that tank is why our bill is huge" cannot both be true, and I was hearing both in the same week. The answer sits in between, and it is easy to work out once you accept the one rule that governs every heated tank: the heater is the bill, and everything else is small change that happens to run all year.
The short answer. A typical 100 litre heated tropical tank in a normally heated UK home costs roughly £130 to £140 a year to run at the July to September 2026 price cap rate of 26.11p per kWh, call it £2.60 a week. Around two thirds of that is the heater, and the rest is the filter, light and air pump ticking over around the clock. A coldwater tank with no heater runs at a third of that. A large marine reef tank, with its powerful lighting and battery of pumps, can cost several times more.
The heater is the bill
A tropical tank holds water at 24 to 26 degrees in a room that is usually somewhere between 17 and 21. Heat leaks out constantly, through the glass, into the cabinet, and above all from the water surface, and the heater's entire job is to replace exactly what escapes. That is the whole cost model. Nothing about the heater itself decides the bill; the tank's heat loss decides it, and the heater merely pays the invoice.
This is worth stating plainly because it trips up almost every newcomer: a 300W heater does not cost more to run than a 100W heater in the same tank. The bigger element simply reaches temperature faster and spends more of its life switched off. When I first put a plug-in monitor on one of my tanks, the pattern that emerged was a rhythm, the heater clicking on for a few minutes, resting, clicking on again, and working noticeably harder in January than in July. The nameplate wattage told me almost nothing; the duty cycle was the story. If you want your own figure, a plug-in energy monitor left on the tank for a full week is the honest way to get it, because a spot reading will catch the heater either flat out or asleep and mislead you in whichever direction you were hoping for.
The surface deserves its own mention. Water does not just conduct heat away, it evaporates, and evaporation is brutally expensive: every litre that leaves the tank as vapour carries roughly 0.6 kWh of heat with it, about 16p at current prices. An open-topped tank losing a litre or so a day to evaporation is quietly spending more on that than on its filter. This is why the single cheapest upgrade in fishkeeping is a lid.
The always-on arithmetic
Everything else on the tank obeys the simplest rule in this whole subject: one watt running around the clock costs about £2.29 a year at 26.11p per kWh. Filters and air pumps run continuously by design, so you just multiply. Lights run on whatever schedule you give them. Here is a typical 100 litre tropical community setup, priced honestly.
| Item | Typical power | Running pattern | Cost per year |
|---|---|---|---|
| Heater (100 to 200W element) | ~40W averaged | Cycles day and night, hardest in winter | ~£91 |
| External canister filter | 12W | Continuous | ~£27 |
| LED light | 15W | 8 hours a day on a timer | ~£11 |
| Air pump | 3W | Continuous | ~£7 |
| Whole tank | ~£137 |
At 26.11p per kWh, the Ofgem price cap rate for July to September 2026. The heater line is a year-round average for a lidded 100 litre tank in a normally heated room; your figure depends on room temperature, the lid, and the temperature you keep the water at. The running cost calculator will do the sum for any wattage and schedule.
Notice what kind of appliance this makes an aquarium. Nothing in it draws much power at any given moment, but nothing in it ever switches off either, which is why a modest tank ends up costing more per year than a washing machine, and appears surprisingly high in our ranking of every appliance. It keeps company with the fridge-freezer and the broadband router, the quiet devices that earn their totals through hours rather than watts.
The room the tank sits in sets the heater's workload
Because the heater only replaces what the room takes away, the same tank costs wildly different amounts in different spots in the house. The temperature gap between water and room is the throttle, and the lid decides how much the surface joins in.
| Situation (100 litres, water at 25°C) | Average heater draw | Per day | Per year |
|---|---|---|---|
| Warm living room (20 to 21°C), lid on | ~30W | 0.7 kWh | ~£69 |
| Typical heated room, lid on | ~40W | 1.0 kWh | ~£91 |
| Cool room (17 to 18°C), lid on | ~60W | 1.4 kWh | ~£137 |
| Cold room or conservatory, open top | 90W or more | 2.2 kWh | £206+ |
Estimated averages at 26.11p per kWh. These are heater cost only; add the filter, light and pump figures from the first table for the whole setup.
The seasons ride the same throttle. In summer a heated tank in a warm room may barely tick over for weeks, while the depths of winter concentrate most of the year's heating cost into a few months, exactly like the house itself. There is a half-comfort here: the heat the tank leaks does warm the room, so in winter it is not entirely wasted. But you are buying that heat at electricity prices, around three and a half times what the same warmth costs through a gas boiler, so a fish tank is a poor radiator even when it is an accidental one.
Coldwater, bigger tanks and reef tanks
Take away the heater and the sums collapse pleasantly. A coldwater setup, goldfish, white cloud minnows and the like, runs on the filter, light and pump alone: about £45 a year for the equipment in the table above, under a pound a week. If the running cost worries you more than anything else, coldwater is the answer, and the fish are no less rewarding for it.
Bigger tanks are gentler than you would fear, because heat loss follows the surface area of the box while the impressive number, the volume, grows faster. A 200 litre tank does not cost twice what a 100 litre costs to heat; something like half as much again is nearer the truth, plus a bigger filter and light. Per litre, big tanks are the cheap ones. Tiny tanks are the opposite: a nano loses heat quickly for its size and its heater cycles hard, though the absolute cost stays small.
Marine reef tanks are the other end of the hobby entirely. Corals demand intense lighting, often 100 to 300 watts running eight hours or more, plus circulation pumps, a protein skimmer and sometimes a chiller fighting the lights in summer. Several hundred pounds a year is normal for a serious reef, several times a tropical community tank of the same size. Anyone pricing one should meter it component by component before committing, the same way you would cost a hot tub, which is the only common household water feature that outspends it.
Cutting the cost without risking the fish
The good news is that the heater responds to exactly the kind of cheap, boring intervention that works on the house itself.
- Fit a lid. It slashes evaporation, which is the most expensive way a tank loses heat, and it keeps the fish in. Glass covers cost a few pounds cut to size. Nothing else on this list comes close per pound spent.
- Insulate the faces nobody looks at. A sheet of polystyrene behind and beneath the tank, and on any side against a cold wall, cuts a worthwhile slice off the heat loss. Aquarists have done this for decades because it works.
- Site it kindly. An internal wall in a heated room, out of draughts, beats a chilly hallway or a conservatory. The scenario table above is really a map of where in the house the tank should live.
- Keep the water at the low end of the species' range. Most community tropicals are perfectly content at 24 rather than 27, and every degree you do not hold costs nothing forever. Check the species, then stop paying for spare degrees.
- Run the light six to eight hours on a timer. The fish do not need more, the algae actively enjoys more, and a timer never forgets. If you are still running old fluorescent tubes, a modern LED unit does the job for a third of the power, £20 or more a year back on a twin-tube hood.
- Never switch the heater off overnight to save money. The room is coldest exactly then, the temperature swing stresses the fish, and the heater spends the morning buying back everything you saved. In a lidded tank the net saving is close to nothing; the vet-level risk is not.
Where does that leave the original argument, the one between "barely anything" and "why the bill is huge"? A heated community tank is a real but modest line on the bill, £2 to £3 a week, less than most people spend on the fish food. It will not explain a frightening bill on its own, and the usual suspects almost always will. But it is one of the few appliances in the house that never, ever clocks off, so it repays ten minutes with a monitor and a sheet of polystyrene better than almost anything its size.
Common questions
How much electricity does a fish tank use?
A typical 100 litre heated tropical tank uses roughly 500 kWh a year, about £130 to £140 at the July to September 2026 cap rate of 26.11p per kWh. Around two thirds of that is the heater. A coldwater tank without a heater uses a fraction of it, around £45 a year for the filter, light and air pump.
Does the heater's wattage decide the running cost?
No. The tank's heat loss decides the cost, and the heater cycles on and off to replace exactly that. A 300W heater in the same tank costs the same to run as a 100W one; it just spends more time switched off. Room temperature, the lid and the water temperature you choose are what move the bill.
Should I turn my aquarium heater off at night?
No. Overnight is when the room is coldest, so the tank would cool fastest, and the morning reheat uses back most of what was saved. The temperature swing also stresses the fish. Insulating the tank and fitting a lid save more, with no risk.
Do aquarium lights use much electricity?
A modern LED unit of 10 to 20 watts on an 8 hour timer costs £8 to £15 a year. Older fluorescent tube hoods use roughly three times the power for the same light, and reef lighting is another matter entirely, often 100 watts and up.
What is the cheapest way to cut an aquarium's running cost?
Fit a lid, which cuts expensive evaporative heat loss. Then insulate the back, base and any cold-facing side with polystyrene, keep the tank in a heated room away from draughts, run the light on a 6 to 8 hour timer, and keep the water at the low end of your species' comfortable range.