Most tropical fish live in water between 74 and 80 °F (23–27 °C), and most homes are cooler than that for at least part of the year. A heater replaces the heat the tank loses to the room through the glass and the surface. Choose one that is too small and it runs constantly while the water drifts down on cold nights; choose one far too big and a stuck thermostat can cook the tank within hours.
Where the "watts per gallon" rule comes from
The familiar advice is 3–5 watts per gallon. It works for a typical tropical tank kept about 10 °F warmer than a living room. But heat loss depends on the temperature difference between water and air: a tank in a 74 °F room needs only a small heater to hold 78 °F, while the same tank in a 60 °F basement needs several times more. So the useful question is: how much must the heater lift the water above the coldest the room ever gets?
A better rule: 0.3 watts per gallon per degree
A lidded glass tank needs about 0.3 W × gallons × temperature lift in °F — or 0.14 W per liter per °C. A 40-gallon tank held at 78 °F in a room that drops to 68 °F needs 40 × 10 × 0.3 = 120 W, so a 150 W heater is the right buy. Adjust for:
- Open tops: add about 25%. Evaporation carries away a lot of heat, and rimless open tanks lose more than any other kind.
- Acrylic: subtract about 15%. Acrylic insulates better than glass.
- Drafts: tanks near exterior doors, windows or air-conditioning vents lose heat faster; size up one step.
Heater sizes by tank and room temperature
| Tank (real water) | Lift 5 °F | Lift 10 °F | Lift 15 °F |
|---|---|---|---|
| 5 gallon (4.7 gal) | 10 W | 25 W | 25 W |
| 10 gallon (9.1 gal) | 25 W | 50 W | 50 W |
| 15 gallon (12.9 gal) | 25 W | 50 W | 75 W |
| 20 gallon high (17.6 gal) | 50 W | 75 W | 100 W |
| 20 gallon long (16.1 gal) | 25 W | 50 W | 75 W |
| 29 gallon (25.1 gal) | 50 W | 100 W | 150 W |
| 40 gallon breeder (39.7 gal) | 75 W | 150 W | 200 W |
| 40 gallon long (34.9 gal) | 75 W | 150 W | 200 W |
| 55 gallon (50.8 gal) | 2 × 50 W | 2 × 100 W | 2 × 150 W |
| 65 gallon (59.6 gal) | 2 × 50 W | 2 × 100 W | 2 × 150 W |
| 75 gallon (69.2 gal) | 2 × 75 W | 2 × 150 W | 2 × 200 W |
| 90 gallon (79.8 gal) | 2 × 75 W | 2 × 150 W | 2 × 200 W |
| 125 gallon (104.4 gal) | 2 × 100 W | 2 × 200 W | 2 × 250 W |
| 150 gallon (138.5 gal) | 2 × 150 W | 2 × 250 W | 3 × 250 W |
Lift is the difference between your target water temperature and the coldest room temperature. Sizes are rounded up to common heater ratings; tanks over 50 gallons are split across two heaters.
One heater or two?
For tanks over about 50 gallons, two heaters of half the wattage are better than one large one:
- Even heat: one heater at each end avoids a warm end and a cool end in long tanks.
- Failure stuck on: a 150 W heater stuck on in a 75-gallon tank raises the temperature far more slowly than a 300 W one, buying you time to notice.
- Failure off: if one heater dies in winter, the other keeps the water from crashing overnight.
Placement
Put the heater where water moves — near the filter outlet or intake, or in the sump — so warmed water is carried around the tank. Mount it diagonally or horizontally near the bottom so the whole element stays submerged as water evaporates. Keep it clear of substrate and decor, which can create hot spots and crack glass heaters. Use a heater guard if you keep large cichlids, plecos or any fish that might rest against it.
Safety and reliability
- Use an external controller on large or valuable tanks. A separate thermostat with its own probe cuts power if the heater's thermostat sticks on.
- Check with a thermometer. Heater dials are often off by 2–3 °F. Set the temperature by the thermometer reading, not the dial.
- Unplug before water changes. A glass heater exposed to air can overheat and crack when the water returns.
- Replace heaters every few years. Thermostats wear out; many keepers replace them on a schedule rather than waiting for a failure.
- Drip loop the cable below the outlet so water running down the cord cannot reach the plug.
Do all fish need a heater?
No. Goldfish, White Cloud Mountain minnows and many native species are coldwater fish that do best at room temperature — a heater can even shorten their lives. Some subtropical fish such as zebra danios tolerate unheated tanks in warm homes. Check the temperature range of each species with the stocking calculator, which also finds the range your fish share.
What does it cost to run?
A correctly sized heater does not run all the time. A 150 W heater that is on about a third of the time uses 150 × 0.33 × 24 × 30 ≈ 36 kWh a month — around $6 at $0.17 per kWh. Lids are the cheapest upgrade: by cutting evaporation they can reduce heating costs noticeably, and they keep jumpers in the tank.
Work out your own tank with the heater size calculator, which takes gallons or liters and °F or °C, or open the worked chart for a 3 gallon, 5 gallon, 10 gallon, 20 gallon, 29 gallon, 40 gallon, 55 gallon, 75 gallon tank.