What Size Heater Works for a 20 Gallon Long Tank?
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What size heater for 20 gallon long aquarium setups? Most hobby guides land between 50 and 150 watts in total, and the right number inside that range depends on how cold your room runs compared with the water temperature you want. A 20 gallon long holds the same volume of water as a standard 20 gallon, so the volume math is identical; the long footprint mainly changes where you put the heater so warm water actually travels the length of the glass. This guide walks through the wattage logic, the two-heater approach, placement, and the mistakes that cause trouble.
Quick answer
- Plan on roughly 2.5 to 5 watts per gallon, which is 50 to 100 watts for 20 gallons, going higher only for a very cold room.
- The bigger the gap between room temperature and your target water temperature, the more wattage you need.
- Two smaller heaters (one source suggests two 25-watt units for a 20 gallon) give a safety margin if one sticks on.
- Always check the real water temperature with a separate thermometer, not the heater dial.
Why tank volume sets the starting point
A heater has to add enough heat to offset what the tank loses to the room. More water means more heat to move, so wattage scales with gallons. One hobby retailer guide suggests starting at about 2.5 to 5 watts per gallon. For a 10 gallon tank that works out to about 25 to 50 watts, and for a 30 gallon tank about 75 to 150 watts. A 20 gallon tank sits between those two, which is why 50 to 100 watts is the usual working range.
The same guide stresses that tank volume is only the first factor. Two keepers with identical 20 gallon long tanks can need different heaters because their homes are different. That is the reason a single number printed on a box is never the whole answer.
How room temperature changes the answer
What matters is the temperature rise: the difference between the room and the water you want. Tropical fish are generally kept at 75 to 80 degrees F (24 to 27 degrees C) according to that guide. If your room sits at 70 degrees F and you want 78 degrees F, you need an 8 degree F rise. If your tank lives in a cold basement or near an air conditioner, the rise is larger and you need more power.
For a 20 gallon tank the guide’s chart gives these ranges:
| Temperature rise needed | Wattage range listed for 20 gallons |
|---|---|
| About 9 degrees F | 50 to 75 watts |
| About 18 degrees F | 75 to 100 watts |
| About 27 degrees F | 100 to 150 watts |
The guide adds that when your tank falls between sizes, you should pick the larger wattage. The page is not perfectly consistent with itself in other places, so treat the figures as a starting range and compare them with the manufacturer’s own sizing chart for the specific heater you are considering.
Things that add or remove heat
- A sunny room needs less heating than a shaded one.
- A cold basement or a spot near air conditioning needs more.
- Lights, filters and powerheads add a little heat of their own, which can lower the wattage you need slightly.
Because a 20 gallon long is wide and shallow, a lid or hood matters too, since an open top lets heat escape from a large surface. This is a practical point rather than a number from the sources, so treat it as something to watch on your own thermometer.
One big heater or two smaller ones?
The retailer guide recommends two smaller heaters instead of one large one. The reasoning is simple: heaters can fail with the contacts stuck “on”, and a single oversized unit stuck on can overheat the whole tank. With two smaller units the worst case is gentler, and if one fails off, the other still keeps the water from dropping sharply. For a 20 gallon tank the guide suggests two 25-watt heaters. It also warns against going above 5 watts per gallon, which would be over 100 watts in a 20 gallon tank.
If you want a belt-and-braces setup, the guide mentions an electronic temperature controller that cuts power when the water gets too hot, with its cutoff set 6 to 8 degrees F above the heater’s setpoint. Whether you add one is a budget and comfort decision; it is not required to run a heater.
Placement in a long tank
The guide advises placing the heater near the filter outflow so the moving water spreads heat through the tank. In a 20 gallon long that means putting the heater at the end where the filter returns water, rather than in a quiet corner where warm water would pool. It also says to mount tubular heaters at about a 45 degree angle, and to confirm the temperature with a separate thermometer placed at the far end of the tank. Comparing two thermometer positions shows quickly whether warmth is spreading evenly.
Before first use, the guide says to let a new heater sit in the tank water for 20 to 30 minutes before plugging it in, and to unplug it before water changes. Never run a heater out of the water. It also suggests replacing heaters every 3 to 5 years and avoiding used ones.
A simple way to choose
- Decide your target temperature for the fish you keep (check the species; tropical fish are often kept between 75 and 80 degrees F).
- Measure your room at the tank’s location, ideally on a cold day.
- Subtract to find the temperature rise.
- Read the 20 gallon row of the chart above, and round up if you fall between ranges.
- Decide whether to split that wattage across two heaters. Many keepers find that comforting.
- Place the heater(s) near the filter output and verify with a separate thermometer.
Common mistakes
- Sizing for a standard 20 and ignoring the room. Volume is only half the question.
- Oversizing. The guide warns against more than 5 watts per gallon.
- Trusting the dial. Heater dials are approximate, so use a thermometer.
- Heater in a dead corner. Water must circulate past it.
- Plugging in a dry heater or leaving it plugged in during a water change.
Related reading on this site
If you are still choosing the tank itself, read our Aqueon 20-gallon long glass tank review. If you are buying a full kit, see the Tetra ColorFusion 20 gallon complete kit review, and check whether the kit already includes a heater before you buy one separately. Planning the stand? The Herture 20 to 29 gallon stand review covers load and storage.
FAQ
How many watts does a 20 gallon long tank need?
Most sources give a range rather than one number. Using 2.5 to 5 watts per gallon gives 50 to 100 watts. A hobby retailer chart lists 50 to 75 watts for a 9 degree F rise, 75 to 100 watts for 18 degrees F, and 100 to 150 watts for 27 degrees F.
Is a 20 gallon long different from a standard 20 gallon for heater sizing?
The water volume is what drives wattage, so the sizing is the same. The long shape mostly affects placement, because you want warm water to circulate the whole length of the tank.
Can I use two 25-watt heaters?
One hobby guide specifically suggests two 25-watt heaters for a 20 gallon as a way to reduce the risk of overheating if one fails stuck on. Whether 50 watts total is enough still depends on how cold your room is.
Where should the heater go?
Near the filter outflow, so moving water spreads the heat. Tubular heaters are often mounted at about a 45 degree angle. Keep the heater fully submerged as the instructions say.
How do I know the heater is working correctly?
Use a separate thermometer and check it at more than one spot. If the reading and the heater setting disagree, trust the thermometer, and consult the heater’s instructions or manufacturer.
Do I still need a heater if the room is warm?
That depends on the species you keep and how stable your room temperature is, including at night. If you are unsure what your fish need, ask a vet or an experienced local fish store.
Bottom line
Start from 2.5 to 5 watts per gallon, which is 50 to 100 watts for a 20 gallon long, move up if your room is cold, and consider splitting the total across two heaters placed near the filter outflow. Verify with a separate thermometer.
