The rule of thumb for aquarium heaters is about 1 watt per litre (roughly 4-5 watts per US gallon) - a little more for cold rooms, a little less for warm ones. So a 100 L tank needs about a 100 W heater. Here's a full size chart and the Fluval and Hygger models that fit.
The wattage rule of thumb
Start at 1 watt per litre to raise and hold tropical temperatures (24-27°C). Bump it up if your room gets cold in winter or is air-conditioned; you can ease down in consistently warm rooms. Is a 100 W heater enough for a 30-gallon (~115 L) tank? Just barely in a warm room - size up to 150 W for a comfortable margin.
Aquarium heater size chart
| Tank volume | Heater wattage | Suggested model |
|---|---|---|
| Up to 40 L (nano) | 25-50 W | Fluval M50 |
| 40-80 L | 100 W | Fluval M100 / E100 |
| 80-120 L | 150 W | Fluval M150 |
| 120-200 L | 200 W | Fluval M200 / E200 |
| 200-300 L | 300 W | Fluval M300 |
| 300-360 L | 500 W | Hygger HG085 500 W |
| Over 360 L | 2 heaters | Two units split across the tank |
Why two smaller heaters can beat one big one
On large or valuable tanks, running two heaters that together add up to the needed wattage is safer: if one sticks off, the other keeps the tank warm; if one sticks on, it can only push half the heat, so the tank overheats more slowly. It also spreads warmth more evenly (see heater safety).
A few sizing tips
- Round up a size if your room is cold, air-conditioned, or the tank is tall.
- Breeding and hospital tanks: size normally, but stability matters most - a display heater (Fluval E) helps.
- India runs on 220-240 V - all the Fluval and Hygger heaters we stock are made for it.
Then set the right temperature for your fish using our temperature guide.
Frequently asked questions
What is the rule of thumb for aquarium heater wattage?
About 1 watt per litre (4-5 watts per US gallon), adjusted up for cold or air-conditioned rooms.
Is a 100 W heater enough for a 30 gallon aquarium?
A 30-gallon tank is ~115 L, so 100 W is marginal - a 150 W heater gives a safer margin, especially in a cool room.
What wattage heater for a 100 litre tank?
Around 100 W in a warm room; step up to 150 W if the room is cold or air-conditioned.
Can a heater be too powerful?
An oversized heater can overshoot if the thermostat sticks - which is why two smaller heaters are safer on big tanks than one very large one.
How do I size a heater for a breeding tank?
Size to volume as normal, but prioritise a stable, accurate heater like the Fluval E-Series with a display.
Do I need a bigger heater in an air-conditioned room?
Yes - AC pulls heat out, so round up a wattage size to hold temperature.
This guide is part of our complete aquarium heater guide - the hub for choosing, sizing and caring for a heater.
Get the right size the first time - shop Fluval and Hygger heaters at Fish Bazaar, delivered across India.
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