Heater size for a 75 gallon tank
A 75-gallon holds about 69 gallons of real water. For a typical room you need about 200 W of heat — best as two 150 W heaters, or in the sump.
Watts needed by room temperature
For 78 °F (25.6 °C) water in a lidded glass tank holding 69.2 gallons of real water:
| Coldest room | Lift | Heat needed | Buy | One heater | Two heaters |
|---|---|---|---|---|---|
| 73 °F (22.8 °C) | 5 °F (2.8 °C) | 104 W | 2 × 75 W | 150 W | 2 × 75 W |
| 68 °F (20.0 °C) | 10 °F (5.6 °C) | 208 W | 2 × 150 W | 250 W | 2 × 150 W |
| 63 °F (17.2 °C) | 15 °F (8.3 °C) | 312 W | 2 × 200 W | over 300 W | 2 × 200 W |
| 58 °F (14.4 °C) | 20 °F (11.1 °C) | 415 W | 2 × 250 W | over 300 W | 2 × 250 W |
"Buy" is the calculator's recommendation: the heat needed rounded up to a common heater size (10, 25, 50, 75, 100, 150, 200, 250, 300 W), split across two heaters because the tank is over 50 gallons. For an open top add about 25%; for acrylic take off about 15% — or use the calculator above with your own temperatures.
One heater or two?
At this size the recommendation is two heaters: 2 × 150 W for a 10 °F lift, each at least half of the 208 W needed. Placed at opposite ends, they heat evenly; if one fails off, the other still supplies half the heat, and if one sticks on it warms the tank half as fast as a single heater would.
If a heater's thermostat sticks on, the worst case (every watt going into the water, none lost to the room) is a rise of about 1.5 °F per hour with one 250 W heater, or 0.9 °F per hour with one 150 W heater of a pair. Real tanks lose heat, so the rise is slower, but it shows why an oversized heater is riskier in a small tank. An external controller cuts power if the water passes your set point.
75 gallons in liters, and the real water
| Tank | Outside dimensions | Rated | Real water |
|---|---|---|---|
| 75 gallon | 48 × 18 × 21 in (121.9 × 45.7 × 53.3 cm) | 78.5 gal (297 L) | 69.2 gal (262 L) |
75 US gallons is 283.9 liters (62.5 imperial gallons). Real water is inside the glass, filled 1 inch below the rim; the heater rule above uses it.
Heating a 75: two heaters, or a sump
At 48 × 18 × 21 inches the 75 has the same length as a 55 but much more water, so the wattage climbs into the range where a single heater would be over 200 W. Two heaters of at least half the total each are the standard approach:
- Display tank: one heater at each end, low and in the flow. The 18-inch depth leaves room to hide them behind rock or plants while keeping water moving past them.
- Sump: if the tank has a sump, put both heaters in the return chamber, where the water level stays constant and the heat is pumped straight back to the display. Make sure they stay submerged if the return pump stops.
- Controller: with 300 W or more installed, use an external controller and set it 1–2 °F above your target as a cut-off.
Cold rooms push the total past 300 W, beyond the largest common single heater; that is where two 200–250 W heaters come in.
How the numbers are worked out
watts = real gallons × lift (°F) × 0.3, about 0.14 W per liter per °C, for a lidded glass tank. For a 10 °F lift that is 3 W per gallon — the low end of the familiar 3–5 W per gallon rule of thumb, which is commonly quoted for heating up to about 10 °F above the room with a lid. That rule would put a 75 gallon tank at 225–375 W. Use the coldest overnight temperature, not the daytime average, and go one size up for drafty rooms. Read the heater size guide for placement and safety.
More about this size: 75 gallon. Other sizes: 3 gal, 5 gal, 10 gal, 20 gal, 29 gal, 40 gal, 55 gal · full chart.
FAQ
How many watts per gallon does a 75 gallon tank need?
About 3.0 W per gallon of real water to hold 78 °F in a 68 °F room (0.3 W per gallon per °F of lift), which rounds up to 2 × 150 W. In a 58 °F room it rises to 2 × 250 W.
What size heater for a 75-gallon tank?
About 200 W of heat for 78 °F water in a 68 °F room, so two 150 W heaters, one at each end or both in the sump.
Where do heaters go in a 75-gallon with a sump?
In the sump return section, fully submerged at the lowest water level, so the warm water is pumped straight to the display.
How many liters is a 75 gallon tank?
75 US gallons is 283.9 liters. Filled about an inch below the rim, the tank holds about 69.2 gallons (262 L) of real water, which is what the heater has to warm.