Heater sizing depends on two things: how much water you are heating, and how far above the room's coldest temperature you need to hold it. A heater sized for the average room temperature will struggle on the coldest night of the year, which is exactly when it matters. The rule aquarists use is roughly two to five watts per litre depending on that temperature gap.
The number that matters is not today's room temperature. It is the lowest the room reaches — overnight in winter, with the heating off. That is when a marginal heater fails.
Work out the gap between that low and your target tank temperature. A tropical community tank around 25 °C in a room that drops to 18 °C overnight needs to bridge seven degrees. In a conservatory or an unheated garage the gap is much larger and the wattage goes up accordingly.
A heater that is slightly oversized cycles less and holds temperature more steadily than one running near its limit. Undersizing is the mistake that costs fish.
Above roughly 200 litres, and in any tank whose stock you care about, two heaters each rated for about half the requirement is the safer arrangement.
The reason is failure mode. A heater that fails off leaves the tank cold, and with two heaters the surviving one slows the drop enough to give you time. A heater that fails on — stuck relay, which does happen — can cook a tank, and a half-sized heater cooks it far more slowly.
Two heaters also distribute warmth more evenly, especially in a long tank where a single heater at one end leaves a cold corner.
Put the heater where water moves past it — near the filter outflow or intake — so warm water is carried around the tank rather than pooling. A heater in a dead corner gives you a warm corner and a cold tank.
Never run a heater with the element out of the water. Switching the heater off before a water change, and back on afterwards, is a habit worth building. Heaters have cracked from being exposed while hot and then refilled with cool water.
Buy a separate thermometer and actually read it. The dial on a heater is a setting, not a measurement, and dials drift. A cheap digital thermometer at the opposite end of the tank from the heater tells you the truth.
Most tropical community fish are kept somewhere around 24 to 26 °C, bettas slightly warmer, and coldwater fish such as goldfish need no heater at all in a normally heated room — they do better cool.
Stability matters more than hitting an exact number. A tank that swings several degrees a day stresses fish more than one held steadily at a temperature slightly off ideal.
Check the requirements of every species in the tank before setting a target. Fish sold together in shops do not always want the same temperature.
Roughly two to five watts per litre, depending on how far above the room's coldest temperature you need to hold the tank. A small gap of a few degrees needs about two watts per litre; a gap of ten degrees or more needs closer to five. Size against the coldest the room gets, not the average.
Two, above about 200 litres or whenever the stock matters. Each rated for roughly half the requirement. If one fails off the other slows the temperature drop, and a half-sized heater that sticks on heats the tank far more slowly than a full-sized one. Two also spread the heat more evenly in a long tank.
Where water moves past it — near the filter outflow or intake — so warmth is carried around the tank. A heater in a still corner warms the corner and leaves the rest cold. Keep it fully submerged at all times.
Usually not. Goldfish are coldwater fish and do better at room temperature than heated. A heater is only needed if the room genuinely drops to uncomfortable extremes or swings sharply.
No. The dial is a setting, not a measurement, and it drifts over time. Buy a separate thermometer and place it at the opposite end of the tank from the heater. Temperature is the parameter most likely to hurt you in a small tank.