Heaters · Adjustable submersible
Fluval M300
Manufacturer-rated for aquariums up to 80 US gal.
Slim 300 W adjustable submersible heater with a 66–86°F maker-published control range.
Consider: At 15 inches it is the longest M heater. Check it fits your water depth at the angle you mount it, and size for your coolest room, not the gallon rating alone.
US version, item A787. Check what’s in the box before you buy.
Tank size Fluval rates it for
Up to 80 US gal
Choose your tank to see where it falls.
Check it against your tank →Specifications that affect fit
- Manufacturer capacity
- Up to 80 US gal
- Power
- 300 W
- Supply voltage
- 120 V / 60 Hz
- Dimensions
- 15 × 1 × 1 in (L × W × H)
To see what it costs to run, enter the watts on its label and your daily hours in the electricity cost calculator.
Before choosing this model
- Size for actual water volume and the required rise above room temperature.
- Verify water temperature with a separate thermometer.
Our buying guides size it for these tanks:
29 gallon40 gallon55 gallon75 gallon
A heater rated for your tank can still fall short in a cold room: see how to size one.
Check it against your tank → Something not working? See the troubleshooting guide.
Included items & upkeep
- heater
- mounting bracket
- suction cups
Plan for the routine work
- Unplug and allow the heater to cool before removal.
- Follow the manual's water-level, placement, and inspection instructions.
To budget for replacement media and supplies, use the first-year cost calculator. For how often and how to clean it, follow the maker’s manual.