An aquarium heater warms your tank water to the temperature your fish need to survive

An aquarium heater is a submersible device that sits in your tank and uses electrical heating elements to raise and maintain water temperature. Most heaters have a thermostat built in—you set your target temperature, and the heater turns itself on and off to hold that level. The heater does not heat continuously; it cycles on when the water cools below your set point and shuts off when the water reaches that temperature.

Different fish species need different temperatures. Tropical fish typically need 76 to 80 degrees Fahrenheit. Goldfish and other coldwater fish do better in the 65 to 72 degree range. If your heater is undersized for your tank, it will run constantly and still fall short. If it is oversized, it can overshoot and cook your fish. Choosing the right wattage and learning how to position and monitor it prevents both problems.

Key Takeaways

  • Most aquarium heaters have a dial or digital display where you set your target temperature, and the thermostat inside turns the heater on and off automatically.
  • A heater sized at 3 to 5 watts per gallon of tank volume will heat most home aquariums without overshooting or running constantly.
  • Place the heater in a spot where water flows past it—near a filter outlet or air stone—so heated water circulates throughout the tank instead of pooling around the heater.
  • Check the water temperature with a separate thermometer every day for the first week after installation to confirm the heater is holding your target temperature.
  • If the heater does not reach your target temperature after running for several hours, the tank may be too large for that heater's wattage, or the room is too cold.

Choosing the right heater size for your tank

Heater wattage is the key to performance. A rule of thumb is 3 to 5 watts per gallon of water. A 20-gallon tank needs a heater between 60 and 100 watts. A 55-gallon tank needs 165 to 275 watts. If you are unsure of your tank volume, measure the length, width, and height in inches, multiply them together, and divide by 231 to get gallons.

Room temperature matters. If your room stays around 70 degrees, aim for the lower end of the range—3 watts per gallon. If your room is cooler or you want to heat a large tank, go higher. A 100-watt heater in a 20-gallon tank in a 65-degree room will work harder than the same heater in a 72-degree room. Most aquarium heaters come in standard sizes: 50, 75, 100, 150, 200, and 300 watts. Pick the size that matches your tank volume and room conditions, not the smallest one that technically fits.

Installing the heater in the right location

Position the heater where water movement is strongest. The best spot is near the outlet of your filter or air stone, so the heater warms water and the current carries that warm water around the tank. Never bury the heater in gravel or hide it behind a decoration where water cannot flow freely past it. The heater needs contact with moving water to sense temperature accurately and to distribute heat evenly.

Most heaters have a suction cup mount or a clip. Attach it to the glass wall at a slight angle—not perfectly vertical—so air bubbles do not get trapped inside the heating tube. If air pockets form, the heater may overheat and fail. Leave at least 2 inches of clearance between the heater and the substrate so water can flow underneath. Keep the heater away from the intake tube of your filter; if the heater pulls in its own hot water, it will not sense the true tank temperature and will overshoot.

Setting the temperature dial or display

Most analog heaters have a dial on top marked in degrees. Turn the dial slowly to your target temperature. For tropical fish, start at 78 degrees. For coldwater fish, set it to 70 degrees. Digital heaters have buttons or a touchscreen; consult the manual for your model, as the interface varies. Do not set the temperature higher than you need—every degree above your target wastes electricity and stresses your fish.

After you set the dial, leave the heater unplugged for 30 seconds, then plug it in. The heater will not start heating when ready; it takes a few minutes for the thermostat to sense the current water temperature and decide whether to turn on. If the water is already at or above your set point, the heater will stay off until the water cools.

Monitoring temperature during the first week

Buy a separate aquarium thermometer—a straightforward glass or digital stick thermometer costs a few dollars and is essential. Place it on the opposite side of the tank from the heater so you read the temperature of the water that has circulated away from the heater. Check the temperature at the same time each day for the first seven days. The water should stabilize within 24 hours and hold steady within 1 degree of your set point.

If the temperature climbs above your target and keeps rising, the heater thermostat may be stuck or faulty. Unplug it when ready and let the tank cool. If the temperature never reaches your target after 8 hours of running, the heater is too small for your tank or your room is too cold. You may need a larger heater or a second heater in a very large tank. Some aquarists use two smaller heaters instead of one large one—if one fails, the other keeps the tank from dropping dangerously.

Troubleshooting common heater problems

If the heater stays on constantly and the water keeps climbing, the thermostat dial may be stuck or the internal thermostat may have failed. Turn off and unplug the heater, wait for the tank to cool, and test the dial by turning it to a lower temperature. If the heater still does not respond, replace it. Do not attempt to repair a heater; they are inexpensive and a faulty one can overheat and harm your fish.

If the heater does not turn on at all, check that it is plugged in and that the outlet works. Some heaters have a small indicator light; if it does not glow, the heater is not receiving power. If the outlet is live but the heater does not light up, the heater is dead and needs replacement. If the heater turns on but the water temperature does not change after several hours, the heater may be too small, or water circulation is poor. Move the heater to a spot with stronger current and wait another 24 hours before deciding it has failed.

Maintaining your heater and replacing it when needed

Heaters last 3 to 5 years on average. Over time, mineral deposits from tap water can coat the heating element and reduce efficiency. You cannot clean inside a sealed heater, but you can extend its life by doing partial water changes every two weeks—this slows mineral buildup. If your heater is more than five years old and your temperature is drifting, replacement is cheaper than troubleshooting.

When you replace a heater, unplug the old one and let it cool for 10 minutes before removing it from the tank. Do not plunge a hot heater into cold water or a cold heater into hot water; the thermal shock can crack the glass tube. Install the new heater in the same location as the old one, set the temperature dial, and monitor the tank for the first week as you did with the original heater.

Frequently Asked Questions

Can I use a heater in a tank with no filter?

Yes, but water circulation will be poor. Without a filter outlet to push warm water around, the heater will create a warm zone around itself and cold zones elsewhere. Add an air stone or small powerhead to move water past the heater and distribute heat evenly throughout the tank.

What happens if the heater breaks while I am away?

If the heater fails and stops heating, the water temperature will drop gradually—usually 1 to 2 degrees per hour depending on room temperature and tank size. Most fish can tolerate a slow drop and survive several days of cooler water. A sudden drop is more dangerous. Using two smaller heaters instead of one large one gives you backup if one fails.

Is it safe to touch the heater while it is running?

No. The glass tube gets very hot—hot enough to burn skin. Never touch a running heater. Always unplug the heater before your hands enter the tank, and wait at least 10 minutes for it to cool before handling it.

Why does my heater keep turning on and off?

That is normal. The thermostat is doing its job—turning the heater on when water cools below your set point and off when the water reaches that temperature. If the heater cycles on and off every few seconds, the tank may be too small for that heater's wattage, or the thermostat is faulty. A heater should cycle roughly every 15 to 30 minutes in a stable tank.

Can I use an aquarium heater in a bowl or very small tank?

Most heaters are designed for tanks of at least 5 gallons. Heaters in very small bowls can overheat the water too quickly and are difficult to control. For bowls under 5 gallons, frequent partial water changes with warm tap water are safer than a heater.