What size heater do I need for my aquarium?
One that can replace the heat the tank loses on the coldest night, with room to spare. That loss has four parts, and the calculator adds them up for your glass and your room instead of multiplying gallons by a number. A standard 75 gallon (48×18×21 in) at 78 °F in a room that drops to 68 °F loses about 224 W with the top open: 75 W through the glass, 6 W through the bottom, 29 W off the surface, and 112 W carried away by the 4 liters that evaporate each day. A heater that runs 60 % of the time at that loss needs to be 373 W — two 200 W heaters. Put a glass lid on the same tank and the loss drops to 108 W and the answer to two 100 W. The chart below runs every standard size both ways.
| Tank | Open: loss | Open: heater | Glass lid: loss | Glass lid: heater | W/gal (lid) |
|---|---|---|---|---|---|
| 5.5 gallon | 36 W | 75 W | 18 W | 50 W | 6.1 |
| 10 gallon | 55 W | 100 W | 28 W | 50 W | 4.9 |
| 15 gallon | 74 W | 150 W | 35 W | 75 W | 4.3 |
| 20 gallon long | 90 W | 2 × 100 W | 42 W | 75 W | 4.1 |
| 20 gallon high | 81 W | 150 W | 43 W | 75 W | 3.8 |
| 29 gallon | 103 W | 2 × 100 W | 55 W | 100 W | 3.5 |
| 37 gallon | 112 W | 2 × 100 W | 64 W | 150 W | 3.2 |
| 40 gallon breeder | 159 W | 2 × 150 W | 72 W | 150 W | 2.9 |
| 55 gallon | 177 W | 2 × 150 W | 93 W | 2 × 100 W | 2.9 |
| 65 gallon | 181 W | 2 × 200 W | 94 W | 2 × 100 W | 2.4 |
| 75 gallon | 224 W | 2 × 200 W | 108 W | 2 × 100 W | 2.4 |
| 90 gallon | 233 W | 2 × 200 W | 117 W | 2 × 100 W | 2.3 |
| 120 gallon | 290 W | 2 × 250 W | 136 W | 2 × 150 W | 2 |
| 125 gallon | 325 W | 2 × 300 W | 151 W | 2 × 150 W | 2.2 |
| 150 gallon | 355 W | 2 × 300 W | 182 W | 2 × 200 W | 2 |
| 180 gallon | 423 W | 2 × 400 W | 192 W | 2 × 200 W | 1.8 |
| 210 gallon | 440 W | 2 × 400 W | 209 W | 2 × 200 W | 1.7 |
Loss from glass area (typical pane thickness for the height), bottom, surface and evaporation; heater = loss ÷ 0.6 duty, rounded up to a size sold, split in two from 200 W. A 10 °F gap; scale the loss with your own gap, roughly linearly. W/gal is what the lidded answer works out to — note how it falls with size.
Heat leaves through surfaces, and volume grows faster than surface. Run the same 10 °F gap across the standard sizes and a lidded 5.5 gallon needs 6.1 W per gallon, a 55 gallon 2.9, and a 180 gallon 1.8 — the small tank needs 3.5× the rule the big one does. "3 to 5 watts per gallon" is a fair description of a lidded 30-to-75 in an ordinary room and a poor one of anything else, and it says nothing at all about the two things that move the answer most: the gap to the coldest night, and the lid.
Does a lid reduce heater size? By about half
Every liter that evaporates takes 0.68 kWh of heat with it — that is what evaporation is. On the open 75 gallon above it is 4 L a day, which is 112 W continuously, 50 % of everything the tank loses — the single biggest item, larger than all the glass. A glass lid cuts evaporation to a sixth and turns the surface into something like a double-glazed window, and the total falls from 224 W to 108 W. That is why the classic charts, written against lidded tanks, look wrong for a rimless open-top: they are. It is also why a reef with a fan blowing across the sump for cooling in summer needs a bigger heater in winter than the same tank with the fan off — and why the evaporation calculator and this page share one engine, so the liters and the watts agree.
Should I use two heaters? The stuck-on arithmetic
Heaters fail two ways, and the stuck-on failure is the one that kills. Where the tank ends up is the same arithmetic as the sizing, run backward: the water climbs until the loss equals the heater's output, room temperature + watts ÷ loss-per-degree. The open 75 loses about 40 W per °C of gap, so a single 300 W heater stuck on takes it to 27.5 °C (81 °F) — unpleasant, survivable. The lidded tank, which needed the smaller heater, is the dangerous one: the same 300 W stuck on takes it to 35.5 °C (96 °F), because the lid that kept the heat in keeps it in. Two 100 W heaters instead, one sticks: 25.2 °C. That is the whole case for two smaller heaters, and it is stronger for a lidded tank than an open one — the opposite of what the smaller lidded heater suggests. A controller that cuts power at a set temperature is the other defense and costs less than the livestock.
| Heater stuck on | Open 75 settles at | Lidded 75 settles at |
|---|---|---|
| 100 W | 22.5 °C · 72 °F | 25.2 °C · 77 °F |
| 150 W | 23.7 °C · 75 °F | 27.7 °C · 82 °F |
| 200 W | 25 °C · 77 °F | 30.3 °C · 87 °F |
| 300 W | 27.5 °C · 81 °F | 35.5 °C · 96 °F |
| 500 W | 32.4 °C · 90 °F | 45.8 °C · 114 °F |
Room at 68 °F; loss linearized at the design gap, so the real runaway is slightly lower (evaporation grows faster than linearly with temperature). Red: above 33 °C, where tropical fish start dying within hours.
Size the heater for the lowest temperature the room actually reaches — the winter night with the thermostat set back, the basement in February, the spare room with the door shut. The average is what sets your electricity bill (below); the extreme is what sets the heater, because a heater that keeps up on average and falls behind at 3 a.m. is a heater that is on 100 % of the time and still losing. Every extra degree of gap is another 40 W of loss on the open 75, 19 W lidded. If you don't know the coldest, use the thermostat setback and subtract a couple of degrees for the floor by the window.
How long does a heater take to warm a tank up?
Water is heavy to heat: 283 liters takes 0.33 kWh per degree Celsius. Warming the open 75 from 68 to 78 °F on a single 300 W heater, losing more and more heat as the gap opens, takes about 10 hours; the lidded tank on its 200 W takes 13. Two things follow. A heater sized only just above the loss takes days to recover from a big cold water change, which is why the duty-cycle headroom is in the sizing. And a heater that can't reach the target isn't slow, it's stuck: with 100 W on the open tank the water stops climbing at 22.5 °C (72 °F), on at full power forever. Type the heater you own into the tool and it tells you which case you're in.
How much does an aquarium heater cost to run?
The heater's rating is not its consumption; the loss is. A 300 W heater on the open 75 draws 300 W only while it's on, and it's on exactly long enough to replace 224 W of loss — 5.4 kWh a day on the coldest night, 2.6 with a lid. On an average day with the room warmer the gap is smaller and so is the bill, roughly in proportion; at a typical 15 ¢/kWh the open tank's worst night costs about 0.8 dollars and its lid pays for itself in a season. The electricity cost calculator takes this loss with your average room temperature, your lights and your pumps and turns it into a monthly figure.
Why won't my tank reach temperature — or why does it swing?
If it never gets there, the heater's output is below the loss at that room temperature, and the tool's "your heater" line shows exactly where it stalls. The usual culprits are a rimless open top (double the loss), a cold draft across the surface (a fan or a window is worth 50 % more loss on the 75), and a room that is colder at night than anyone measured. If it swings, that is the thermostat's hysteresis meeting a heater with no flow around it: a heater in a dead corner switches off when its water is warm and the far end is not. Put it in the flow, near the filter return, low in the tank. Neither problem is fixed by a bigger heater — the first by a lid or a second heater, the second by moving it. The heater is also the cheapest accelerator you own when cycling a tank and the one piece of quarantine equipment that is not optional. If your room is warmer than the target, this is the chiller calculator's problem, and the same loss numbers feed it.