Aquarium Heater Size Calculator
Calculate the correct heater wattage for your aquarium based on tank size, room temperature, and your target temperature for tropical or cold water fish.
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Aquarium Heating Guide
What should I know about 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.
What's the key thing to understand about 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 glass and virtually unbreakable, making them a worthwhile upgrade for larger tanks or households with boisterous fish or curious pets nearby.
What should I know about 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.
What's the key thing to understand about 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.