Holds a set point
The heater switches on when the water drops below its setting and off when it gets there. It can add heat, but it cannot cool, and steady matters more than the exact number.
Heating & temperature
How big a heater do I need?
A heater's real job is to keep the water steady. Size it for how cold the room gets, put it where water moves, and check it with a separate thermometer. Most tropical tanks do well somewhere around 76-80°F.
What it does
A heater makes up the gap
The heater only has to cover the difference between your room and your target temperature. That gap decides the size.
Know your range
Most tropical freshwater and reef tanks sit around 76-80°F. Check the range for the fish you keep.
- Bettas are tropical and need a heater
- Goldfish and other coldwater fish often skip one; ask about yours
- Keep day-to-night swings small, roughly 1-2°F
Cold rooms need more watts
The colder the room, the more heat the tank loses and the harder the heater has to work.
- Basements, outside walls, and drafty windows run cold
- Open tops lose heat through evaporation
- Small tanks cool faster for their size
Heat comes from gear too
Lights, pumps, and a hot room can push a tank above target. A bigger heater will not fix that.
- Watch the thermometer on hot days
- Warm water holds less oxygen
- Add surface movement if the tank runs warm
Rough guide
Start with about 3-5 watts per gallon
Use the low end in a warm, steady room and the high end in a cold one. This is only a starting point; check the manufacturer's sizing chart for your heater.
About 30-50 watts
Small tanks lose heat quickly for their size, so lean toward the top of the range if the room gets cool.
About 60-100 watts
One heater usually covers this size. Go larger if the tank sits near a window, an outside wall, or a cold floor.
About 165-275 watts
Many keepers split this total between two smaller heaters for more even heat and a safety margin.
Two or more heaters
A 75-gallon works out to about 225-375 watts. Two 150-watt heaters instead of one 300-watt heater is a common split.
Size up, then add a backup
Garages and unheated basements can need more than 5 watts per gallon. Pair extra wattage with a temperature controller.
Shop controllers
Setup tips
Placement matters as much as watts
A well-sized heater in a dead corner still heats unevenly. Put it where the current spreads the warmth.
Split into two heaters
On tanks around 55 gallons and up, two smaller heaters are often safer than one big one.
- Heat spreads more evenly
- If one sticks on, it overheats the tank more slowly
- If one fails, the other slows the cooling
Put it in moving water
Near the filter outflow or intake, or in a sump's return chamber, flow carries heat around the tank.
- Keep the water above its minimum line mark
- Do not bury it in gravel or sand
- Leave a little space from glass and rock
Let it settle first
Let a new heater sit in the water before plugging it in. Many instructions say 15-30 minutes.
- Use a drip loop on every cord
- Plug into a GFCI-protected outlet
- Adjust in small steps over a day or two
Use a heater guard
A guard keeps fish from resting against a hot heater and protects the heater from bumps.
- Helpful with large or bottom-resting fish
- Make sure water still flows around it
Check and protect
Trust a thermometer, not the dial
Heater dials can be off by a few degrees. Check what the water is actually doing and protect the tank from a heater that fails.
Read it every day
Place a separate thermometer away from the heater so it shows what the fish feel.
- Glass or digital probe both work
- Compare two thermometers now and then
- Log the reading with your other tests
Add a safety cutoff
A temperature controller uses its own probe to cut power if the water gets too warm, which protects against a heater stuck on.
- Check it is rated for the heater's wattage
- Keep its probe away from the heater
- Use its alarm if it has one
Inspect while you clean
Give heaters a quick look during regular maintenance.
- Unplug and let it cool before wiping off film
- Look for cracks or water inside the tube
- Replace it if it cannot hold temperature
Unplug before water changes
A heater running in air can overheat and crack, or shatter when water hits it again.
- Unplug heaters before draining any water
- Plug back in once they are fully covered
- Let a heater cool in the water before removing it
Common mistakes
What usually goes wrong
Most heater problems come from placement, trust in the dial, or water changes.
Leaving it on during a water change
Unplug first. A heater exposed to air can crack, and glass heaters can shatter when cool water hits them.
Hiding it behind rock
Without flow, it warms one corner and shuts off early. Move it into the filter or return current.
Read about flowCranking the heat
Change temperature slowly, about 1-2°F a day at most. Ask before raising heat to treat disease.
Parasites and ich