Heaters, sized by arithmetic instead of a rule of thumb

This is the hub the site was really built around, because it is the one where everybody publishes the same rule of thumb and nobody publishes the working. You will read “three to five watts per gallon” on every aquarium site there is. Nobody tells you which end of that range applies to your room, why a 25W heater is rated for six gallons and a 50W for fifteen when that is not a straight line, or what to do when your answer falls between two sizes that exist.

So here is the whole thing, in public. 5 US gal rated, less 15% for substrate and fill line, is 4.3 gal of water. A 78F target minus a coldest-room 68F is a 10F lift. 4.3 x 0.35 x 10 = 15.1W required. Nothing is sold at 15.1W, so the answer is the 25W.

Where the constant comes from

The 0.35 in that sum is not folklore. It is read off Fluval’s own published P Series ratings. Fluval rates each heater to a tank capacity, and separately publishes that a lift of more than 12°F over ambient room temperature means you need additional heating capacity. That makes every rating a triple - watts, gallons, and a maximum delta:

• P10: 10W up to 3 gallons → 10 ÷ (3 × 12) = 0.278 W per gallon per °F
• P25: 25W up to 6 gallons → 25 ÷ (6 × 12) = 0.347
• P50: 50W up to 15 gallons → 50 ÷ (15 × 12) = 0.278

We use 0.35, the most conservative of the three, because undersizing a heater is the failure that kills fish and oversizing one only makes it cycle more often. The full method page shows the derivation, the assumptions and the table for every common tank size and room temperature.

What to buy, if you read nothing else

For a 5-gallon betta tank in a house that stays above about 64°F: a 25W submersible heater. For a 3-gallon in a warm room: 10W. For a 10-gallon: 50W. Those three answers cover the overwhelming majority of desk tanks in the United States, and all three come straight out of the sum above rather than out of a rule of thumb that happens to be roughly right.

How this category divides

Preset heaters hold a fixed temperature, usually somewhere between 76 and 78°F, with no dial. That is the correct temperature for a betta, so the lack of a dial is not the drawback it sounds like - right up until you need to raise the tank for a sick fish or drop it in a heatwave.

Adjustable heaters give you a dial. The dial is also the part that drifts, and a cheap adjustable heater set to 78 that actually holds 82 is worse than a preset that honestly holds 76. If you buy adjustable, you are buying a separate thermometer as well - and you should own one either way.

Controller heaters put the thermostat outside the tank, on a probe. They are the accurate option and they cost two or three times as much. In a nano tank the case for one is genuinely weaker than in a big tank, because a 5-gallon has so little thermal mass that the failure mode you care about is not drift, it is the heater sticking on.

The one arrangement we would avoid entirely is a heater with no visible indicator light. In a small tank you want to be able to glance across the room and see that it is cycling.

The mistake buyers make

Sizing for the average room, not the coldest one. A heater that holds 78°F on a mild evening and gives up at 3am in January has not done its job, and the fish spends the winter at 73 getting slowly sicker. Every figure on this site is computed against the lowest temperature your room actually reaches - which, for most American houses, Aquarium Co-Op puts around 67 to 69°F.

The second mistake is trusting the heater’s own setting. Buy a thermometer. A five-dollar glass one on the opposite end of the tank from the heater will tell you more about your tank than the dial ever will.