7.9
Fluval P10 Submersible Heater, 10W
Ten watts, rated to 3 US gallons, 5.3 inches long. It is the smallest heater in this class that publishes a capacity AND a maximum lift, which is the pair of numbers you actually need. In a 3-gallon in a 72-degree room our sum says 5.5W; the P10 clears that with room to spare. In a 64-degree room it does not, and Fluval's own 12-degree note says the same thing.
| Specification | Value | Source |
|---|---|---|
| Wattage (120 V / 60 Hz) | 10W | Fluval P Series published specifications |
| Rated aquarium capacity | up to 3 US Gal / 10 L | Fluval P Series published specifications |
| Dimensions | 5.3" L x 0.78" W x 0.59" H (13.5 x 2 x 1.5 cm) | Fluval P Series published specifications |
| Maximum lift over room temperature | 12F / 8C - beyond that Fluval states additional heating capacity is likely necessary | Fluval P Series published guidance |
| Temperature setting | Preset - not adjustable | Fluval P Series published specifications |
The sum for this one
Covers a 3-gallon down to about a 68F room, and no further.
3 US gal rated, less 15% for substrate and fill line, is 2.6 gal of water. A 78F target minus a coldest-room 68F is a 10F lift. 2.6 x 0.35 x 10 = 9.1W required. Nothing is sold at 9.1W, so the answer is the 10W.
What it does well
- Publishes wattage, capacity AND maximum lift - almost nothing else in this class does
- 5.3 inches hides behind almost any hardscape
- Fully submersible, so it can lie flat under the substrate line
What it does not
- Preset, so you cannot dial a degree either way
- Genuinely underpowered in a cold room
- No display, so you are buying a separate thermometer regardless
Your room drops below 68F. At a 3-gallon and a 64F room the arithmetic wants 12.7W and you have 10, which is how a tank sits at 74F all winter.