Aquarium heater wattage, by the math

A 20-gallon tank from 68°F to 78°F is 100 watts on this calculator. Manufacturer charts for that volume do not agree.

Heater size for a 29-gallon tank, from the Aqueon chart, is on what size heater for a 29 gallon tank.

Why two heaters

At 40 gallons and up, this calculator adds a two-heater note: each unit is half the rounded wattage, stepped up to the next 25 watts, and at least 100 watts. Two units at opposite ends keep heat from sitting in one corner. If one fails, the other is still plugged in. The page does not state how many degrees the surviving heater holds.

Thermostat and placement

Put the heater near the filter outflow so heated water mixes. Put the thermometer at the opposite end of the tank from the heater. A stick-on strip reads the glass, not the water. During a water change, unplug the heater before the element is in air. Hot glass in air can crack.

Power-outage planning

A stocked tropical tank cools during a winter outage. Wrap the tank, or run the heater from backup power. Sizing that backup is a battery-and-runtime calculation, which is why the footer links to GeneratorSizer and Battery Bank Sizer. This page does not state how many hours a tank stays warm.

A worked 20-gallon example

Section added October 6, 2026. The block below is the heater function on the calculator, with the inputs written out. Watts per gallon on this site are a planning estimate, not a published standard. Open the Heater size tab and enter the same numbers.

The rise is 78 − 68 = 10°F. The function uses 2.5 watts per gallon at a rise of 5°F or less, 3 watts up to 9°F, 4 watts up to 15°F, and 5 watts above that. Ten degrees uses 4. Raw watts are 20 × 4 = 80. Standard sizes checked in order are 25, 50, 75, 100, 150, 200, 250, 300, 400, and 500. The first size at or above 80 is 100 watts. A second heater is noted only at 40 gallons and up, so this 20-gallon run is one unit.

Put the same 20 gallons next to three manufacturer charts. They do not agree, and this page does not average them. Every band is in the heater and filter size chart.

• Aqueon's heater wattage guidelines list 50 watts for 5–20 gallons and 100 watts for 15–40 gallons. Twenty gallons is inside both bands. That one-page chart does not state a room temperature or a degrees-of-rise.
• EHEIM thermocontrol (types 3611–3619) calls its table standard ratings for closed aquariums at an ambient room temperature of 21°C (70°F) and a desired water temperature of up to 26°C (79°F). The 75 watt row is 15.8–26.4 US gallons. Twenty gallons sits in that row, not in the 100 watt row (26.4–39.6 US gallons).
• Fluval E100 is a 100 watt heater for aquariums up to 30 US gallons (120 L). The E-series manual says those volumes are approximate for water up to 26°C (78°F) with a room temperature of 20°C (68°F), and that a lower room temperature may require a higher wattage. Twenty gallons is inside the E100 "up to 30" line under that stated room and water temperature. E200 is 200 watts up to 65 US gallons, and E300 is 300 watts up to 100 US gallons.

For this one set of inputs, the calculator and the Fluval E100 conditions (68°F room, about 78°F water) both land on 100 watts. EHEIM's 75 watt row includes 20 gallons only under its own 70°F-room, up-to-79°F-water rating. Aqueon includes 20 gallons in two wattage bands at once. Use the chart that states the room you actually have. If the room is colder than the chart's assumption, the Fluval manual says a higher wattage may be required. Do not copy the 100 watt result into a 55°F basement.

A common mistake

Reading "20 gallons" on a box and buying the smallest wattage that mentions 20. On the Aqueon chart, 20 is the top of the 50 watt band and also inside the 100 watt band. The calculator's 100 watt result is for a 10°F rise from 68°F, not for every room.

Limits

These charts are manufacturer ranges for intact, submerged heaters. They are not a diagnosis of a heater that has cracked, run dry, or stopped controlling. If the tank will not hold temperature, or the heater case is damaged, unplug it and replace it. An aquatic veterinarian is the person to call when fish are sick after a temperature crash, not a wattage table. Backup power for an outage is the topical reason the footer links to GeneratorSizer and Battery Bank Sizer.

FAQ

What wattage does this calculator return for 20 gallons from 68°F to 78°F?

100 watts. The rise is 10°F, the function uses 4 watts per gallon for a rise above 9°F and at most 15°F, the raw load is 80 watts, and the next standard size at or above 80 is 100. The two-heater note starts at 40 gallons, so this example is one heater.

Which manufacturer band includes a 20-gallon tank?

Aqueon lists 20 gallons in both the 50 watt band (5–20 gallons) and the 100 watt band (15–40 gallons). EHEIM thermocontrol's 75 watt row is 15.8–26.4 US gallons, for a 70°F room and water up to 79°F. Fluval's E100 is 100 watts for aquariums up to 30 US gallons, at about 78°F water and a 68°F room. Those are three different charts. They are not the same number.

Compiled with AI assistance from the sources below. Spot an error? Email jpanasuk@gmail.com.

Sources

• Aqueon. Heater Wattage Guidelines. https://www.aqueon.com/-/media/project/oneweb/aqueon/files/us/educational/aq_heater_wattage_chart.pdf. Accessed October 6, 2026.
• EHEIM. thermocontrol operating manual, types 3611–3619. https://www.eheim.com/media/pdf/c7/65/0c/3611-3619_thermocontroldPSgHh7gyBpLG.pdf. Accessed October 6, 2026.
• Fluval. E100 product page, and E-series heater manual (volumes stated for about 78°F water and a 68°F room). https://fluvalaquatics.com/us/shop/product/e100-electronic-heater-100w-up-to-30-us-gal-120-l. Manual PDF: https://www.aquasabi.de/cdn/fluval/Fluval_E-Serie_Heizer_DE_EN.pdf. Accessed October 6, 2026.

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