The standard rule for aquarium heater size is 1 watt per litre of water in a normally heated room, and 1.5 to 2 watts per litre if the room gets cold, for example a bedroom where the heating goes off at night or an unheated room in winter. So a 60 litre tank in a warm living room needs about a 75 W heater, while the same tank in a cold room needs 100 W or more. The real variable is not the tank, it is the gap between your room temperature and the temperature your fish need. Below: a sizing table you can read in five seconds, how to measure that gap, when to use two heaters instead of one, and which heater type fits your setup.
Contents
- How many watts per litre does an aquarium heater need?
- Aquarium heater size chart by tank volume
- Why room temperature matters more than tank size
- In-tank, external or built into the filter: which heater type?
- Should I use one heater or two?
- How do I set up an aquarium heater correctly?
- Frequently asked questions
How many watts per litre does an aquarium heater need?
Use these three rules of thumb, based on how much warmer the tank has to be than the room:
- Up to 5°C warmer than the room (heated flat, tank at 25°C, room at 21°C): about 1 W per litre is plenty.
- 5 to 10°C warmer (room drops to 16 to 18°C at night): 1.5 W per litre.
- More than 10°C warmer (unheated room, garage, cold winter nights): 2 W per litre, ideally split between two heaters.
Small tanks lose heat faster relative to their volume, so for nano tanks under 30 litres round up rather than down. A heater that is slightly too strong is not dangerous as long as its thermostat works, because it simply switches off sooner. A heater that is too weak runs non-stop and still cannot hold the temperature on the coldest night.
Flat, shatterproof heater for 10 to 25 L. Electronic thermostat accurate to ±0.25°C, display with the current water temperature, and an automatic lower night temperature via a light sensor.
Aquarium heater size chart by tank volume
Find your tank volume, then pick the column that matches your room in winter:
| Tank volume | Warm room (20°C+) | Cool room (16 to 19°C) | Cold room (under 16°C) |
|---|---|---|---|
| 20 to 30 L | 25 to 50 W | 50 W | 50 to 75 W |
| 40 to 60 L | 50 to 75 W | 75 to 100 W | 100 to 150 W |
| 80 to 100 L | 100 W | 150 W | 200 W |
| 150 L | 150 W | 200 to 250 W | 300 W (2 × 150 W) |
| 200 to 250 L | 200 to 250 W | 300 W | 2 × 200 to 250 W |
| 300 L+ | 300 W (or 2 × 150 W) | 2 × 200 to 300 W | 2 × 300 W or more |
Tank volume here means the real water volume. A "100 litre" tank with substrate, rocks and wood usually holds 80 to 85 litres, which gives you a little extra safety margin.
Which of our heaters fits your tank?
The manufacturer ratings below assume a heated room. If your room gets cold at night, go one size up.
| Tank volume | Heater | Price |
|---|---|---|
| 10 to 25 L | Aquael Ultra Heater 25 W, Day & Night | €32.99 |
| 60 to 100 L | Aquael Platinium Heater 100 W | €24.99 |
| 60 to 100 L, with day and night mode | Aquael Ultra Heater 100 W, Day & Night | €39.99 |
| 90 to 150 L | Aquael Platinium Heater 150 W | €25.99 |
| 200 to 300 L | Aquael Platinium Heater 300 W or 2 × Platinium 150 W | €29.99 |
| Large tanks and fish rooms | Schego Heater 600 (titanium) + Schego TR2 thermostat | €86.40 + €97.75 |
Why room temperature matters more than tank size
A heater does not "heat a volume", it replaces the heat the tank loses to the room. The colder the room, the faster the tank loses heat, and the harder the heater has to work. That is why the same 100 litre tank can be perfectly fine with 100 W in October and struggle on a January night when the central heating switches off at 23:00 and the room drops to 15°C.
Before you buy, measure the coldest point of the day in the room where the tank stands, usually early morning. A thermometer that shows both water and room temperature makes this a one-glance job, and it also tells you straight away if a heater has failed.
A few things that make a big difference to heat loss:
- A lid or cover glass. Open-top tanks lose a lot of heat through evaporation.
- Position. Tanks next to a window, an outside wall or a door lose heat much faster.
- Strong surface agitation. Great for oxygen, but it also increases heat loss.
Not sure what temperature your tank should be at? Most tropical community fish and planted tanks do well at 24 to 26°C, and our guide to temperature in planted aquariums goes deeper.
Shows water and room temperature on one display, mounts on the outside of the glass, no probe or cable in the tank.
In-tank, external or built into the filter: which heater type?
All three do the same job. The difference is where the heater sits and how the tank looks.
| Type | Pros | Cons | Best for |
|---|---|---|---|
| In-tank heater | Cheapest, easy to install and replace, works with any filter | Visible in the tank, takes up space | Most community and beginner tanks |
| Filter with integrated heater | Nothing visible in the tank, very even heat, one device instead of two | Higher upfront price, filter and heater are replaced together | Aquascapes and planted tanks with a canister filter |
| Inline external heater | Nothing visible in the tank, works with your existing canister filter, high power | Needs matching hose size, slightly more installation | Large aquascapes and display tanks |
Canister filter with a built-in 100 W heater for tanks of 60 to 100 L. Water is warmed as it flows through the filter, so there is no heater in the tank at all. 3-year warranty.
For bigger tanks the same idea comes as the OASE FiltoSmart Thermo 300 (up to 300 L) and the OASE BioMaster² Thermo 250. If you already run a canister filter with 16/22 mm hoses, the Inline External Aquarium Heater PRO (0 to 1000 W) fits straight onto the hose.
Should I use one heater or two?
For tanks over about 200 litres, two smaller heaters are better than one big one. Split the total wattage between them (for example 2 × 150 W instead of 1 × 300 W) and place them at opposite ends of the tank. You get two advantages:
- More even temperature across a long tank, with no cold corners.
- A built-in safety net. If one heater fails "off", the other keeps the tank from crashing overnight. If one sticks "on", each heater alone is too weak to cook the tank quickly, which gives you time to notice.
The second point is the one people underestimate. A heater stuck on is one of the few equipment failures that can wipe out a whole tank in a few hours, which is why a separate thermometer is not optional.
For 90 to 150 L. Electronic thermostat (±0.4°C), built-in thermometer, ceramic heat sink against overheating and one-button setting. Two of them at opposite ends cover a 250 to 300 L tank with a built-in backup.
For very large or high-value tanks, you can go one step further: a titanium heater such as the Schego Heater 600, switched by an external controller like the Schego TR2. The TR2 reads the water through its own probe, controls heating or cooling up to 2000 W, and keeps its settings after a power cut. Note that the Schego 600 has no thermostat of its own and must be used with the TR2.
How do I set up an aquarium heater correctly?
Step 1: Place it in the flow
Put the heater near the filter outlet or intake so warm water is carried around the whole tank. Never bury it in the substrate or wedge it behind rocks.
Step 2: Let it acclimatise before switching on
Put a glass heater in the water and wait 15 to 20 minutes before plugging it in, so the glass adjusts to the water temperature. Always check the minimum water level marked on the heater.
Step 3: Set the temperature, then check with a thermometer
Set your target and give it a few hours to stabilise. Then trust the thermometer, not the dial: built-in thermostats can be off by a degree or more.
Step 4: Unplug it during water changes
When the water level drops, a heater that is still on can overheat in air and crack. Unplug it first, plug it back in once the tank is full again.
Step 5: Check it every season
Before winter, check that the heater actually switches on and off (most have an indicator light) and look for cracks or discolouration. A heater more than a few years old is cheap insurance to replace.
Get your tank ready for winter
The right heater and a reliable thermometer cost less than replacing a single group of fish.
Shop aquarium heaters →Frequently asked questions
What size heater do I need for a 60 litre aquarium?
In a heated room, a 50 to 75 W heater is enough. If the room drops below about 18°C at night, go for 100 W.
Can an aquarium heater be too powerful?
A slightly oversized heater is fine as long as the thermostat works, because it switches off sooner. A heavily oversized heater becomes a risk if it ever sticks on, so do not go far beyond about 2 W per litre, and use two smaller heaters on big tanks.
What temperature should my aquarium be?
Most tropical community fish and planted tanks do well at 24 to 26°C. Some species prefer warmer water (bettas, discus) and some cooler (medaka, many shrimp), so always check the needs of what you keep.
Do I need a heater if my flat is heated?
For tropical fish, almost always yes. Even heated homes drop several degrees at night or when the heating goes off, and it is that drop, not the daytime temperature, that stresses fish.
How do I keep my tank warm during a power cut?
Cover the tank with a lid and wrap it in blankets or polystyrene to slow heat loss, and avoid opening it. A well-covered tank in a heated room usually loses only a few degrees over several hours.