What Size Heater Does a 40 Gallon Tank Need?
40 gallons is listed under two Aqueon bands at once: 100 watts (15-40) and 150 watts (30-55).
Full breakdown: aquarium heater wattage guide.
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The figures
| Figure | Value | Source |
|---|---|---|
| Question size | 40 gallon tank | Aqueon |
| First band that contains 40 gallons | 100 watts for 15-40 gallons | Aqueon |
| Second band that contains 40 gallons | 150 watts for 30-55 gallons | Aqueon |
| 50 watt band | 5-20 gallons; 40 is above 20 | Aqueon |
| 200 watt band | 50-75 gallons; 40 is below 50 | Aqueon |
| 300 watt band | 65-90 gallons; 40 is below 65 | Aqueon |
How to size it
This is an edge case the chart creates on purpose: neighboring bands share gallons. Aqueon lists 100 watts through 40 gallons and 150 watts from 30 gallons upward. A tank of exactly 40 gallons is on both lines.
The next band does not join them. 200 watts is listed for 50-75 gallons, and 40 is below 50. 300 watts starts at 65. The 50 watt band ended at 20. So the overlap to explain is only 100 watts and 150 watts, and it exists because 40 is inside the shared region that runs from 30 gallons through 40 gallons. The chart does not say to add the watts together, and this page does not add them. It names both listings. The shopping links are a 100 watt heater and, as the next band up, a 150 watt heater.
No Seriously Fish minimum in this set lands between 29 and 40 gallons. The largest minimum that still fits is about 21 US gallons (zebra danio and molly). The next published volume is the angelfish at about 53, which is above 40, so moving from a 29 to a 40 does not unlock angelfish, discus, or oscar. Dwarf gourami volume is 56 liters. 56 ÷ 3.785 = 14.80 US gallons, still under 40, and that profile lists no temperature.
Species whose published minimum fits this tank
Same fit list as a 29 gallon tank. A 40 does not clear any additional minimum in this set. Temperatures are the species ranges.
| Species | Published minimum | Temperature | Source |
|---|---|---|---|
| Betta (Betta splendens) | about 11 US gallons | 72-86°F | Seriously Fish |
| Neon tetra (Paracheirodon innesi) | about 14 US gallons | 70-77°F | Seriously Fish |
| Guppy (Poecilia reticulata) | about 11 US gallons | 63-82°F | Seriously Fish |
| Zebra danio (Danio rerio) | about 21 US gallons | 64-77°F | Seriously Fish |
| Bronze cory (Corydoras aeneus) | about 19 US gallons | 70-81°F | Seriously Fish |
| Platy (Xiphophorus maculatus) | about 14 US gallons | 68-79°F | Seriously Fish |
| Molly (Poecilia sphenops) | about 21 US gallons | 70-82°F | Seriously Fish |
| Bristlenose pleco (Ancistrus sp.) | about 19 US gallons | 77-86°F | Seriously Fish |
| German blue ram (Mikrogeophagus ramirezi) | about 14 US gallons | 81-86°F | Seriously Fish |
| Dwarf gourami (Trichogaster lalius) | 56 liters = 14.80 US gallons | Not listed | Seriously Fish |
| Cherry barb (Puntius titteya) | about 14 US gallons | 68-81°F | Seriously Fish |
| Harlequin rasbora (Trigonostigma heteromorpha) | about 14 US gallons | 70-82°F | Seriously Fish |
Still above a 40 gallon tank:
| Species | Published minimum | Temperature | Source |
|---|---|---|---|
| Angelfish (Pterophyllum scalare) | about 53 US gallons | 75-86°F | Seriously Fish |
| Discus (Symphysodon aequifasciatus) | 255 liters = 67.37 US gallons | Not listed | Seriously Fish |
| Oscar (Astronotus ocellatus) | about 143 US gallons | 68-82°F | Seriously Fish |
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Accessory: a digital thermometer. The chart lists two wattages for this gallon size, and the thermometer is how you see what the water actually does. Shop a digital aquarium thermometer on Amazon
Related pages
- Stocking calculator
- What size heater a 55 gallon tank needs
- What size heater a 29 gallon tank needs
- What size tank a bristlenose pleco needs
FAQ
What size heater does a 40 gallon tank need?
Aqueon lists 40 gallons under two bands: 100 watts for 15-40 gallons, and 150 watts for 30-55 gallons. The 200 watt band starts at 50 gallons and does not include 40.
Why do two wattages both include 40 gallons?
The 100 watt band ends at 40 gallons and the 150 watt band starts at 30 gallons. The shared stretch is 30 through 40 gallons, and 40 is the top of that stretch. Aqueon does not say to add 100 and 150 together.
Does a 40 gallon tank fit an angelfish?
No. Seriously Fish lists the angelfish minimum at about 53 US gallons, which is above 40. Discus converts to 67.37 US gallons and the oscar minimum is about 143 US gallons. Both are above 40.
Sources
- Aqueon, Heater Wattage Guidelines. https://www.aqueon.com/-/media/project/oneweb/aqueon/files/us/educational/aq_heater_wattage_chart.pdf. Accessed Oct 2, 2026.
- Seriously Fish, Betta (Betta splendens). https://www.seriouslyfish.com/species/betta-splendens/. Accessed Oct 2, 2026.
- Seriously Fish, Neon tetra (Paracheirodon innesi). https://www.seriouslyfish.com/species/paracheirodon-innesi/. Accessed Oct 2, 2026.
- Seriously Fish, Guppy (Poecilia reticulata). https://www.seriouslyfish.com/species/poecilia-reticulata/. Accessed Oct 2, 2026.
- Seriously Fish, Zebra danio (Danio rerio). https://www.seriouslyfish.com/species/danio-rerio/. Accessed Oct 2, 2026.
- Seriously Fish, Bronze cory (Corydoras aeneus). https://www.seriouslyfish.com/species/corydoras-aeneus/. Accessed Oct 2, 2026.
- Seriously Fish, Platy (Xiphophorus maculatus). https://www.seriouslyfish.com/species/xiphophorus-maculatus/. Accessed Oct 2, 2026.
- Seriously Fish, Molly (Poecilia sphenops). https://www.seriouslyfish.com/species/poecilia-sphenops/. Accessed Oct 2, 2026.
- Seriously Fish, Bristlenose pleco (Ancistrus sp.). https://www.seriouslyfish.com/species/ancistrus-sp/. Accessed Oct 2, 2026.
- Seriously Fish, German blue ram (Mikrogeophagus ramirezi). https://www.seriouslyfish.com/species/mikrogeophagus-ramirezi/. Accessed Oct 2, 2026.
- Seriously Fish, Dwarf gourami (Trichogaster lalius). https://www.seriouslyfish.com/species/trichogaster-lalius/. Accessed Oct 2, 2026.
- Seriously Fish, Cherry barb (Puntius titteya). https://www.seriouslyfish.com/species/puntius-titteya/. Accessed Oct 2, 2026.
- Seriously Fish, Harlequin rasbora (Trigonostigma heteromorpha). https://www.seriouslyfish.com/species/trigonostigma-heteromorpha/. Accessed Oct 2, 2026.
- Seriously Fish, Angelfish (Pterophyllum scalare). https://www.seriouslyfish.com/species/pterophyllum-scalare/. Accessed Oct 2, 2026.
- Seriously Fish, Discus (Symphysodon aequifasciatus). https://www.seriouslyfish.com/species/symphysodon-aequifasciatus/. Accessed Oct 2, 2026.
- Seriously Fish, Oscar (Astronotus ocellatus). https://www.seriouslyfish.com/species/astronotus-ocellatus/. Accessed Oct 2, 2026.