Aquarium Heater Size Calculator

Aquarium Heating Guide

Heater Sizing Rule

The standard rule of thumb is 1 watt per litre for tanks in rooms that are 5–10°C below the target temperature, and up to 2 watts per litre for larger temperature differences or unheated spaces. For a 100-litre tank in a room at 18°C targeting 26°C (8°C rise), a 100–150W heater is appropriate. Always slightly oversize — a heater working at 70% capacity runs cooler and lasts longer than one at full load.

Heater Types

Submersible heaters (glass or titanium): the most common type, placed fully underwater at the back or side. Inline heaters: fitted to the external filter return — heat the water as it re-enters the tank, giving the most even temperature distribution. Substrate heaters: cables under the gravel, providing gentle bottom heat and encouraging plant root growth. For planted tanks, a combination of a submersible heater and substrate cable is often recommended. Titanium heaters are more durable than gla

Temperature and Fish Health

Temperature stability matters more than the exact temperature. A swing of 2–3°C in a day stresses fish far more than being kept at 24°C versus 26°C. Use a digital thermometer with an alarm to detect heater failures early. Heaters fail in both directions — stuck on (overheating) or stuck off (cooling). A second heater at a lower setting (set 1–2°C below the primary) acts as a backup and prevents catastrophic overheating if the primary heater fails on.

Tropical Temperature Guidelines

Most tropical community fish (tetras, corydoras, livebearers): 24–27°C. Discus: 28–30°C. Cichlids: varies widely by species (22–30°C). Betta splendens: 24–28°C. Goldfish and other cold water fish: 10–20°C — a heater is not needed in a typical indoor setting but a thermometer is still essential to monitor seasonal fluctuations. Avoid placing tanks near windows (temperature swings), air conditioning vents, or radiators.