Buying advice
Aquarium Heater Size Guide
Aquarium heater size depends on two things: how much water you have and how far above room temperature you need to keep it.
A 20-gallon aquarium kept at 78°F in a 73°F room does not have the same heating job as a 20-gallon aquarium kept at 78°F in a 63°F basement. Tank size is the same. Heat loss isn't.
For most aquariums, start by finding the difference between the desired water temperature and the lowest normal room temperature.
If the tank needs to stay at 78°F and the room usually bottoms out at 68°F, the heater needs to handle about a 10°F temperature rise.
Aquarium heater wattage by tank size
This table is a practical starting point for common aquarium sizes. The wattages are approximate. Tank shape, room temperature, an open top, strong surface movement, drafts, filtration and other parts of the setup can change how much heat is needed.
| Aquarium size | About 5°F above room | About 10°F above room | About 15°F above room |
|---|---|---|---|
| 5 gallons | 50W | 50W | 50W |
| 10 gallons | 50W | 50W | 50W |
| 20 gallons | 50W | 100W | 100W |
| 30 gallons | 100W | 100W | 150W |
| 55 gallons | 150W | 150W | 200W |
| 75 gallons | 200W | 200W | 300W |
| 100 gallons | 300W | 300W | 300W |
| 120 to 125 gallons | 400W total | 400W total | 400W total |
| 150 to 210 gallons | 600W total | 600W total | 600W total |
The table uses heater sizes that are commonly sold, so the numbers don't increase smoothly with every gallon or degree. A heater is controlled by a thermostat and cycles on and off rather than constantly delivering its full rated wattage.
Once the temperature difference gets much above 15°F, a generic table becomes less useful. Check the sizing guidance for the exact heater you're considering. A cold basement, garage, exterior wall or frequently opened door can make a heater work much harder than the gallon count suggests.
Open-top tanks can also lose more heat than covered tanks.
Room temperature matters more than people think
Use the coldest normal room temperature, not the temperature beside the aquarium when you happen to check it.
Suppose you want 78°F water.
A room that stays around 73°F gives the heater a 5°F rise.
A room that falls to 68°F gives it a 10°F rise.
At 63°F, the heater has to make up 15°F.
That difference can move the same aquarium into another heater size.
This is also why the familiar watts-per-gallon rules need some context. Around 3 to 5 watts per gallon is a useful starting range when the aquarium only needs to run about 10°F above the room. It isn't a promise that any 5-watt-per-gallon heater can hold any temperature in any room.
Smaller aquariums also have less thermal mass, so temperature can move faster when conditions change.
Preset vs. adjustable aquarium heaters
A preset heater has a fixed thermostat. Many are designed around the upper 70s, but the actual preset temperature depends on the model. Check it rather than assuming every preset heater targets the same temperature.
Preset heaters are appealing when their fixed temperature already matches the aquarium. There is no dial to set, and they work well for simple setups where the desired temperature isn't likely to change.
The limitation is obvious. You can't choose another temperature.
An adjustable heater lets you select a target within the heater's operating range. That makes more sense when the fish or other livestock need a temperature different from the preset value, or when you simply want control over the setting.
Adjustment markings aren't the same thing as measured water temperature. Keep a separate aquarium thermometer in the tank even if the heater has a numbered dial or electronic display.
For many aquariums, adjustable is the more flexible purchase. Preset makes sense when its fixed target matches the tank and simplicity is the priority.
Submersible heaters and other heater designs
A fully submersible heater sits inside the aquarium and can operate below the water surface. This is the normal choice for most freshwater and saltwater tanks.
Many current submersible models can be mounted vertically or horizontally, although this is something to verify on the exact heater. Some designs have specific water-level or orientation requirements.
Submersible heaters are simple, widely available and easy to place in moving water. Their main downside is that the heater itself, its mounting hardware and its cord are inside the display.
Filter-integrated heaters put the heating element inside or alongside the filtration system. The aquarium looks cleaner because there is less equipment in view, and the filter moves water past the heater continuously. Compatibility becomes more restrictive because the heater has to work with the filter.
Inline heaters are installed outside the aquarium in the water line, usually with an external filtration system. They can keep the display free of heater hardware, but hose size, flow requirements and installation all have to match.
There are also small flat heaters and other compact designs intended for nano aquariums. Some simply add a limited amount of heat rather than giving the same temperature control as a thermostatic adjustable heater. Read the operating description carefully when shopping for a very small tank.
For a normal aquarium without a reason to hide the heater elsewhere, a fully submersible thermostatic heater is usually the simplest choice.
When two aquarium heaters make sense
Two heaters become useful when one heater can't comfortably supply the required wattage.
Large tanks are the obvious case. Once an aquarium reaches roughly 100 gallons, splitting the total heating capacity between two heaters is common. A 400-watt requirement, for example, can be handled with two 200-watt heaters rather than trying to find one suitable 400-watt unit.
Long aquariums can benefit as well. Putting heaters in different areas can help prevent one end of the tank from being warmer than the other.
A large temperature difference between the room and the aquarium is another reason to consider two heaters.
There is also some redundancy if one heater stops producing heat. The remaining heater may slow the temperature drop until the problem is noticed.
Two heaters do not make the aquarium fail-safe. Thermostats and heating elements can fail in different ways, so the water temperature still needs to be monitored.
When using two heaters, think in terms of total wattage. Two 150-watt heaters provide 300 watts of installed heating capacity.
Where to place an aquarium heater
Put the heater where water moves past it.
Near a filter outlet, filter intake or circulation pump is usually better than a quiet corner behind decorations. Moving water carries the heat through the aquarium instead of allowing a warmer pocket to form around the heater.
In a long aquarium using two heaters, placing them in different parts of the tank can improve temperature distribution.
Follow the heater's mounting instructions. A model that permits horizontal placement can often sit low on the back wall, where it stays submerged during normal water-level changes. Don't assume every heater can be mounted this way.
Keep the heater clear of anything that blocks water movement around it.
A separate thermometer is more useful when it measures the tank rather than the warm water immediately beside the heater. Putting the thermometer some distance away can make uneven heating easier to spot.
Heaters should also be unplugged before a water change exposes them to air. Follow the manufacturer's cooling instructions before removing a recently operating heater from the water.
What matters when comparing aquarium heaters
Wattage comes first. A heater with features you like is still the wrong heater if it can't keep up with the tank.
Then look at the temperature control. A preset heater should state its fixed target. An adjustable heater should give its available temperature range.
Control accuracy is a separate specification. Two adjustable heaters may both have a 78°F mark on the dial while using different thermostats and claiming different levels of accuracy.
The manufacturer's aquarium-size rating is useful, but don't compare gallon ratings without also checking wattage. There is no reason to assume every brand uses identical room temperatures, aquarium conditions or safety margins when assigning a tank-size rating.
Physical size can decide the purchase in a shallow aquarium, sump chamber or tightly aquascaped tank. Check the heater's actual length and the mounting space it needs.
Construction varies too. Glass, protected glass and shatter-resistant polymer housings are all available. Tanks containing large fish, turtles or animals likely to strike equipment give durability more weight than a quiet planted aquarium might.
Check whether the heater is fully submersible, whether horizontal mounting is allowed, and whether it is approved for freshwater, saltwater or both.
Safety features are worth comparing by what they actually do. Dry-run protection, overheat shutoff and other automatic shutoff systems are not all the same feature.
An indicator light tells you when a basic heater is heating. More elaborate models may show the measured temperature or warn when the water moves outside a set range.
Warranty length belongs in the comparison too, especially when two heaters otherwise have similar specifications.
The useful comparison isn't “100W heater A versus 100W heater B.” It is whether both heaters can handle the required temperature rise, fit the tank, provide the control you want and include the protections you care about.
Once you know the wattage and heater type you need, compare offers within those requirements. A lower price on the wrong wattage isn't much of a deal.
The deal module should show products that match the heater specifications being compared on this page. Prices, discounts, stock and retailer availability should come from current site data rather than being written permanently into the guide.
Sizing first. Shopping second.