How many degrees warmer than the room?
The first number to work out is the rise: your target water temperature minus the coolest the room gets. Fish kept at 78°F in a room that drops to 68°F need a 10°F rise; the same tank in a 75°F room needs only 3°F. Volume and a lid matter too, which is why makers publish charts instead of one watts-per-gallon figure.
Enter your water, room and target temperatures to read Aqueon’s and Fluval’s charts side by side. Our heater pages list each model’s rated tank range and size, so you can check it fits the depth of your tank.
Use the heater maker’s own chart
Each maker’s chart is written for its own heaters, so read the chart of the heater you are buying; two heaters with the same wattage from different makers can be rated for different tanks. Some charts change the size with the rise above room temperature, others do not.
Where a maker’s own pages disagree, take the more careful one. The Aqueon Pro 100 is rated for tanks up to 40 gallons, yet for a 40 gallon held at 78 °F in a 70 °F room Aqueon’s own chart asks for 150 W, so check the chart against your room and the temperature you need rather than the box.
For big tanks Aqueon’s chart sometimes calls for two heaters; it gives the wattage for each, so buy two of that size. Two heaters spread the heat, and if one fails the other slows the cooling, but they do not protect you from a heater stuck on: checking a thermometer every day is how you catch one. Fluval’s guide adds that a lid, the water movement and a cold spot near the tank all change how hard a heater works.
Preset or adjustable temperature?
A preset heater holds one temperature, usually about 78°F; an adjustable one has a dial or a display you set. Either way, check it with a separate thermometer for a day or two after you set it, since dials are often a degree or two out.
Place it where water moves past it, usually near the filter outlet, at the angle and depth its manual shows. A heater too long for a shallow tank cannot be fixed by choosing a smaller wattage: check its length against your water depth first.
What it costs to run
A heater uses its rated watts only while it is heating; its thermostat switches it on and off. How much of the day it runs depends on how much colder the room is than the water, the lid and the season. Counting it at full watts for 24 hours gives the most it could cost, not what it usually does.
The electricity cost calculator asks for a low and a high share of the day, so you see the range. A plug-in energy meter tells you your own heater’s real figure over a week.
Taking it out safely
Unplug the heater and leave it in the water for the time its manual gives (30 minutes for Aqueon’s) before you lift it out, so the glass cools where the water can take the heat. Never run it in air, and keep the water above its minimum mark. A heater with a crack, water inside or a damaged cord is replaced, not repaired.
Plan the plugs so you can unplug the heater without reaching across the water. Keeping a tank warm in winter