Does an Aquarium Need a Heater

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Does aquarium need heater? It depends on the fish. The reference article on aquarium heaters says most tropical freshwater and marine aquariums are kept between 22 and 30 degrees C (72 to 86 F), and a heater is the device that warms the water to that range. If your fish are tropical and your room is cooler than that, you need one. If your fish are suited to cooler water, you may not. This page goes through what the sources say, what they leave out, and how to decide without guessing.

Quick answer

• Tropical fish: a heater is normally part of the setup, since tropical tanks are kept at 22 to 30 C (72 to 86 F).
• Check what temperature your species needs first; the heater is a means to a number, not a rule.
• A thermostat in the heater holds the set temperature by switching the element on and off.
• Larger tanks can use two or more heaters, so one can keep the tank warm if the other fails.

What a heater actually does

The Wikipedia article on aquarium heaters describes the most common type, the glass immersion heater. A glass tube holds a heating element wound around a ceramic or glass insert. Some models contain sand, and most, though not all, can be fully submerged. A built-in thermostat switches the element on and off to hold the set temperature. The thermostat is often a bimetallic strip: two metals with different expansion rates bend the strip as the temperature rises, which breaks the circuit at the target temperature. The setpoint can be adjusted, higher-end units may use microchip-based thermostats, and a small indicator light often shows when the heater is running.

So a heater is not a steady source of heat. It cycles on and off to hold a number. That is why the setting and the thermometer matter more than the wattage printed on the box.

When you probably need one

If you keep tropical freshwater fish, the 22 to 30 C range from the reference is the target. A betta is a tropical species, and the Wikipedia article on the Siamese fighting fish describes wild air temperatures of about 15 to 40 C, but it describes those as habitat figures and not care guidance. For your own fish, look at a care guide for the species and set the heater to that number. Our guide Best Aquarium Heater for Betta Fish: 6 Safe Picks compares listings for small tanks, and the model page Hagen Marina Betta Submersible Aquarium Heater covers one specific heater.

Indoor rooms change through the day and the year. A tank in a cool room, near a window or in a room that is not heated at night will move with the room unless something holds it steady. That is the case for a heater with a thermostat.

When you might not

If your fish are comfortable at room temperature, and your room stays in their range, a heater is optional. Our post on whether rasboras need a heater looks at one such case and what the sources say. The key is that you must know the temperature range of the species, and check that your room really stays inside it. The sources we have do not give a species list, so check a care guide for yours.

What the sources do not cover

The heater article does not address wattage selection, placement, safety or the specific requirements of tropical versus coldwater fish. That means we cannot give you a watts-per-gallon rule from these sources, and we will not invent one. For wattage, read the maker’s guidance for your tank size on the product page. Our reviews of individual models show what each listing states: Hitauing aquarium heater, 100W model and range and Seaoura small aquarium heater, 50W. If you want several heaters side by side, see hygger vs DaToo: 6 aquarium heaters compared.

Other ways heat is delivered

The reference describes two alternatives to the immersion heater:

• Undergravel heating: a flexible heating cable is buried beneath the substrate. It is popular in heavily planted tanks.
• Undertank heating: heating mats placed beneath the tank. The article notes glass conducts heat poorly, so this method is less efficient.

For most small home tanks the immersion heater is the common choice, simply because it is easy to add and to replace.

Why a second heater is mentioned

The same reference says a small tank can use one heater, a larger tank can use two or more, and a second unit can keep the tank warm if the other fails. That is a reasonable way to think about risk: a single heater is a single point of failure. The article does not say how to split the wattage between two units, so follow the instructions that come with the heaters you buy.

A practical way to decide

1. Find the temperature range for your species in a trustworthy care guide.
2. Put a thermometer in the tank for a few days with no heater and read it at different times of day.
3. If the reading falls outside the range, you need a heater. If it stays inside, you may not, but keep watching through the seasons.
4. Choose a heater with an adjustable thermostat, set it to your target and confirm with the thermometer.
5. Consider a backup unit for larger tanks.

A heater also does not replace good filtration; for that side see UPETTOOLS aquarium products: filters, lights, heater and tools compared.

FAQ

Do all aquariums need a heater?

No. The sources describe heaters in the context of tropical tanks kept at 22 to 30 C. Fish suited to cooler water may not need one, but you must check the species.

What temperature should a tropical tank be?

The reference says most tropical freshwater and marine aquariums are kept between 22 and 30 C (72 to 86 F). The exact number for your fish depends on the species.

How does a heater know when to stop?

A built-in thermostat switches the element on and off. Many use a bimetallic strip that bends as the temperature rises and breaks the circuit at the setpoint.

Can I use two heaters in one tank?

Yes, the reference says larger tanks can use two or more, and a second unit can keep the tank warm if the other fails.

Are there heaters other than the glass tube kind?

Yes: undergravel heating cables and undertank heating mats, though the article says mats are less efficient because glass conducts heat poorly.

What wattage do I need?

The sources do not give a rule. Follow the maker’s guidance for your tank size, and ask the shop or a fish vet if you are unsure.

Bottom line

If your fish are tropical and the tank would otherwise drift below their range, get a heater with an adjustable thermostat and a thermometer to check it. If your species is happy at room temperature, confirm that with a thermometer before skipping it.

Sources

Aquarium heater (Wikipedia); Siamese fighting fish (Wikipedia)