How heater sizing works
A heater does not "heat the tank to size" — it continuously replaces the heat the tank loses to the room. That loss depends on three things: how much water there is, how far the water is from room temperature, and how exposed the tank is. The calculation behind the tool above is:
watts ≈ k × litres × ΔT ΔT = target water temp − room temp · k ≈ 0.12 (covered) / 0.2 (open) / 0.3 (drafty room) The tool then adds ~15% headroom for thermostat drift and heat-up after water changes, and rounds up to a wattage you can actually buy. The coefficient k is calibrated against published manufacturer sizing charts — the results land inside the ranges you see on heater packaging, while staying explainable instead of magic.
One heater or two?
Above roughly 200 W of total heating, run two heaters instead of one big unit. Three reasons: if one thermostat ever sticks on, the other unit shares rather than doubles the load; long tanks heat evenly from both ends instead of warm-here-cold-there; and no single heater needs an unusual wattage. Place them at opposite ends, near filter flow, fully submerged. The calculator switches its recommendation automatically once you cross that threshold.
Nano tanks and big temperature gaps
Two situations deserve extra care. In tanks under ~20 litres, temperature swings fast and an oversized heater can overshoot before a thermostat reacts — prefer a low-wattage heater (or an external thermostat) and keep a thermometer in view. With temperature gaps beyond ~15 °C, no reasonable heater holds the tank cheaply: a lid, insulation behind and under the tank, or a warmer room will do more than more watts.
Typical temperature bands
Target temperature comes from your livestock, not from the heater. Common bands (the same presets the calculator offers):
| Livestock | Water temperature | Heater? |
|---|---|---|
| Community tropical | 24–26 °C | Yes |
| Betta | 26–28 °C | Yes |
| Neon & cardinal tetras | 23–26 °C | Yes |
| African cichlids | 24–27 °C | Yes |
| Discus | 28–30 °C | Yes |
| Cherry shrimp | 20–24 °C | Yes |
| Guppies & endlers | 22–28 °C | Yes |
| Platys & swordtails | 20–26 °C | Yes |
| Tiger barbs | 22–27 °C | Yes |
| Kuhli & zebra loaches | 22–27 °C | Yes |
| Bristlenose plecos | 20–27 °C | Yes |
| Angelfish | 24–28 °C | Yes |
| Snails | 18–28 °C | Yes |
| Goldfish | 18–22 °C | Usually not |
| Koi & pond fish | 10–24 °C | Usually not |
| White cloud minnows | 16–22 °C | Usually not |
| Axolotl | 14–18 °C | Usually not |
Cool-water species (goldfish, white cloud minnows, axolotls) are the exception: in most homes they need no heater at all — and for axolotls heat is actively dangerous.
Wattage cheat sheet
Standard heater sizes you will find in stores: 25 W, 50 W, 75 W, 100 W, 150 W, 200 W, 300 W, 500 W. The calculator always rounds up to one of these — or splits the total across two or more units for large tanks.
Placement and safety
- Position near the filter outflow so heat circulates; a heater behind plants in dead water reads the wrong spot's temperature.
- Keep the heater fully submerged to the marked line and off the substrate; use a guard with digging fish or shrimp.
- After a water change, let the glass equalise before power returns — a hot heater meeting cold air can crack.
- Always run a separate thermometer; thermostat dials are approximate and drift with age.
Frequently asked questions
What size heater do I need for my aquarium?
A good starting point is the heat your tank loses to the room: roughly 0.1–0.3 watts per litre per °C of gap between the water and the room, depending on whether the tank is covered, open-topped or in a drafty spot. A covered 100 litre tank holding 25 °C in a 20 °C room typically needs a 75–100 W heater. Our calculator works this out from your real numbers instead of guessing.
Is the "5 watts per gallon" rule correct?
It is a rough screening rule for typical room temperatures, not physics. It ignores the biggest factor — how cold the room is. A tank in a 26 °C room needs almost no heat at any size, while the same tank in a 16 °C basement needs far more than 5 W per gallon. Use the rule as a sanity check; size from volume, temperature gap and exposure.
Can a heater be too big for a tank?
Yes, in two ways. An oversized heater in a nano tank can cook the livestock quickly if the thermostat sticks on, and small tanks swing in temperature much faster. A modestly larger heater than needed is fine and even desirable (it runs gentler duty cycles), but a 300 W heater in a 10 litre bowl is asking for trouble. For big tanks prefer two or more heaters over one giant unit.
Should I use one heater or two?
Above roughly 200 W of total heat, prefer two heaters (for example 2 × 200 W instead of 1 × 300–500 W). Two units give redundancy if one thermostat fails, even out warm and cold ends on long tanks, and keep each unit at a comfortable size. Place them at opposite ends, ideally near filter outflow so heat circulates.
Do goldfish need a heater?
Usually no. Goldfish are comfortable at roughly 18–22 °C and do fine at room temperature in most homes. If your room stays very cold in winter, a small heater can hold the tank at a stable temperature, but the main risk for goldfish is summer overheating — above about 24–25 °C they need cooling, aeration and reduced feeding.
How long does a tank take to heat up?
Warming 100 litres by 5 °C with a 75 W heater takes many hours — our estimate is about 8 hours, and real tanks lose heat while heating so it can take longer. Never add livestock before the tank has held its target temperature steadily for a day or two. A heater is for holding a temperature, not for fast heat-up.
Where should I place the heater?
Near the filter outflow or in an area of strong flow so heated water circulates instead of pooling around the thermostat. Keep it fully submerged and off the substrate (or use a guard with digging fish and shrimp), and give the glass a few minutes to equalise before switching on after a water change.
Next steps
Heater size starts from water volume: if you have not measured your tank yet, the aquarium volume calculator gives you the real number in litres and gallons. The guides section covers what is coming next — filter flow, water changes and stocking.