An aquarium heater keeps water at a steady temperature that matches what your fish need to survive
Most tropical fish die or stop eating when water temperature drops below 75°F. A heater is a device that sits in your tank, plugged into an outlet, and turns on and off automatically to hold the water at your target temperature. You set the dial or digital display to your desired temperature, place the heater in the tank, and it does the work for you. The heater itself is usually a glass or metal tube about 6 to 12 inches long, with a control dial on top or a digital readout on the side.
Without a heater, your tank temperature will drift with your room temperature. In winter, unheated tanks can drop to 60°F or lower, which stresses fish and invites disease. In summer, some tanks climb above 82°F, which also harms most fish and reduces oxygen in the water. A heater solves both problems by maintaining a narrow, safe range.
Key Takeaways
- Choose a heater sized to your tank volume — a 50-watt heater works for tanks up to 20 gallons, and larger tanks need 100 watts or more.
- Place the heater near a water return or filter outlet so heated water circulates throughout the tank instead of staying in one spot.
- Set the dial or digital display to your target temperature, usually 76°F to 78°F for most tropical fish, and let it stabilize for a few hours before adding fish.
- Check the temperature with a separate thermometer daily for the first week, then weekly after that, to confirm the heater is holding steady.
- Never touch a heater that is plugged in or still hot, and always unplug it before doing tank maintenance.
Choosing the right wattage for your tank size
Heater wattage determines how fast and how high the water can heat. A general rule is 3 to 5 watts per gallon of water. A 20-gallon tank needs a 60- to 100-watt heater; a 40-gallon tank needs 120 to 200 watts. If your room is very cold or you live in a climate where winter temperatures drop sharply, choose the higher end of that range.
A heater that is too small will run constantly and may never reach your target temperature. A heater that is too large will overshoot and create temperature swings, which stress fish. Most aquarium heaters come in standard sizes — 50, 75, 100, 150, 200, and 300 watts — so match the closest size to your tank volume.
If you are unsure of your tank volume, measure the length, width, and height in inches, multiply them together, and divide by 231. That gives you gallons. A 20-gallon long tank is 30 inches long, 12 inches wide, and 12 inches tall; a 40-gallon breeder tank is 36 by 18 by 18.
Where to place the heater in your tank
Position the heater near the filter outlet or return flow so that warm water spreads through the tank instead of pooling around the heater. If your filter has an intake and return tube, place the heater within 6 inches of the return so the filter's current carries heat to the rest of the tank. In a tank with no filter, place the heater in a corner where water naturally circulates.
Keep the heater away from the substrate (gravel or sand) at the bottom. Heaters sitting directly on the bottom can crack if the glass touches hard surfaces, and they heat the substrate unevenly. Most heaters have suction cups on the back; use them to attach the heater to the glass wall, about 2 inches above the gravel.
Never place a heater under a light or in direct sunlight. Heat from the light will interfere with the heater's ability to sense water temperature accurately, and the heater may shut off too early or stay on too long.
Setting the temperature and letting it stabilize
Most tropical fish thrive between 76°F and 78°F. Bettas, gouramis, and tetras do well at 76°F. Discus fish and some cichlids prefer 80°F to 82°F. Check the species requirements for your fish before you set the dial. If you keep a mix of species, aim for 78°F as a middle ground.
Turn the heater on and set the dial or digital display to your target temperature. Do not add fish yet. Let the heater run for 4 to 6 hours, then check the temperature with a separate thermometer. The heater should be holding steady within 1°F of your target. If the temperature is still rising or falling after 6 hours, the heater may be too small or too large, or it may be faulty.
Once the temperature is stable, leave the heater on and wait another 24 hours before adding fish. This gives the heater time to prove it can hold the temperature overnight and through a full day-night cycle.
Monitoring temperature daily and weekly
Buy a separate thermometer — a straightforward glass stick thermometer costs a few dollars and is reliable. Place it on the opposite side of the tank from the heater so you are reading the temperature of the water that has traveled farthest from the heat source. Check the thermometer every morning for the first week. The temperature should not swing more than 2°F from your target.
After the first week, check the thermometer once or twice a week. If the temperature starts to drift — rising slowly or dropping — the heater may be failing. A heater that runs constantly without raising the temperature is broken and should be replaced. A heater that does not turn off at all will eventually boil the tank.
In summer, if the tank temperature climbs above 80°F without the heater running, you may need a chiller or fan to cool the water. Most heaters only heat; they do not cool. If your room stays above 82°F for weeks at a time, a heater alone will not solve the problem.
Safety and maintenance steps
Never touch a heater while it is plugged in or while the water is still warm. Heaters get hot enough to burn skin. Always unplug the heater before you reach into the tank for maintenance, water changes, or cleaning. Wait at least 10 minutes after unplugging before touching the heater itself.
Do not let the heater sit in air. If the water level drops below the heater during a water change, unplug it when ready. A heater running in air will overheat and crack. Always refill the tank to its normal level before turning the heater back on.
Check the heater cord for cracks or damage once a month. If the cord is frayed or the plug is loose, replace the heater. A damaged heater is a shock hazard and a fire risk. Most heaters last 3 to 5 years before the heating element fails or the thermostat becomes unreliable.
Troubleshooting common heater problems
If the heater is on but the temperature is not rising, the heater may be too small for your tank, or the room temperature may be much colder than your target. Try raising the dial 2°F and wait 6 hours. If the temperature still does not rise, the heater is likely faulty and needs replacement. Do not leave a broken heater in the tank.
If the temperature is rising above your target and the heater will not turn off, the thermostat is stuck. Unplug the heater when ready and replace it. A heater that will not shut off can cook your fish and boil the tank dry.
If the heater is making a humming or buzzing sound, it may be failing internally. Most heaters are silent when working normally. A noisy heater should be replaced within a few days. If the heater cracks or leaks, remove it at once and do a partial water change to dilute any contaminants that may have entered the tank.
Frequently Asked Questions
Can I use a heater in a tank with no filter?
Yes. A heater works independently of a filter. Without a filter, place the heater in a corner where water naturally moves, and stir the tank gently with a stick once a day to mix warm and cool water. A tank without a filter will need more frequent water changes, but the heater will still maintain temperature.
What happens if I unplug the heater for a few hours?
The temperature will drop slowly, usually 1°F to 2°F per hour depending on room temperature and tank size. Most fish tolerate a temporary drop of 5°F to 10°F for a few hours without harm. Plug the heater back in as soon as you finish maintenance. Do not leave it unplugged overnight.
Do I need a heater if my room is always warm?
If your room stays between 76°F and 78°F year-round, you may not need a heater. However, most homes cool down at night or in winter. A heater is cheap insurance against temperature swings that stress fish. Even in warm climates, a small heater is worth having.
Can two heaters in one tank cause problems?
Two heaters can work together if each is set to the same temperature and sized appropriately. A 40-gallon tank with two 100-watt heaters will heat faster and more evenly than one heater alone. Set both dials to the same temperature and place them on opposite sides of the tank. If one fails, the other keeps the tank warm while you replace it.
How do I know if my heater is accurate?
Compare the heater's dial reading to a separate thermometer placed in the same spot. If they differ by more than 2°F, the heater's thermostat is drifting. You can adjust some heaters by turning a small screw on the back, but most cannot be recalibrated. If the error is large, replace the heater.