How to Pick the Right Heater Wattage

Written 2026-10-28

A heater’s wattage needs to match your tank’s actual water volume in liters, not its label. In our catalog, 25W is sized for a nano tank (~20L), 50W for a small tank (~40L), 100W for a medium tank (~90L), 200W for a large tank, and 300W for an extra-large tank. We ran the simulator on a 90cm standard tank (actual water volume 170.1L) fitted with a 25W heater, in an unheated winter room at 16°C, for 90 days. The water temperature plateaued at 20.57°C and never got closer to the 25°C target, so 10 neon tetras started dying from day 34 and all were gone by day 53. Swapping in a 100W heater, same tank, same room temperature, got the water up to 24.89°C and all 10 fish survived. The wattage itself isn’t the problem — it’s a mismatch with tank volume, and that mismatch only shows up once the room gets cold.

Wattage has to match your tank’s volume

Each of our five catalog heaters is sized for a certain water volume. Fitting a smaller heater to a bigger tank is cheaper and less visible, but its heating capacity really is limited to that smaller volume.

What a mismatch does — four scenarios

We put 10 neon tetras in the same 90cm standard tank (170.1L), changing only the heater and room temperature, adding the fish once cycling was done (day 30), and watched for 90 days.

A 25W heater in a 170.1L tank gets stuck at 20.57°C in a cold room — well under both the 25°C target and neon tetras’ tolerated range (22–27°C). Even with perfect water quality, cold stress alone killed the first fish by day 34 and all 10 by day 53. Swap in a 100W heater under the same conditions and the water reaches 24.89°C, and all 10 survived the full 90 days.

A warm room hides the problem

The same undersized heater doesn’t show any trouble when the room is warm. With the room at 22°C instead of 16°C, that same 25W heater in the 90cm tank got the water to 24.96°C and all 10 neon tetras were fine. That’s why a heater that worked all spring and summer can suddenly cause trouble once winter comes and the heating gets turned down — the heater didn’t break, it was simply undersized for that tank all along. On the other hand, fit that same 25W heater to the tank it’s actually sized for (a 20 cube, 6.8L of actual water), and even in a cold 16°C room it reached 24.61°C, keeping 4 cherry shrimp fine for the full 90 days. The wattage isn’t bad — it only becomes a problem when it doesn’t match the tank.

Takeaway

When picking a heater: ① check your tank’s actual water volume (liters after subtracting headspace, not the size printed on the box), ② use the table above to match that volume to a wattage, and ③ when in doubt, size up (we didn’t find any case in this simulator where an oversized heater caused problems in a smaller tank). If your room loses a lot of heat in winter, it’s safer to leave some margin.

The button below runs the second example from this guide — a 90cm standard tank · 100W heater · 16°C winter room · 10 neon tetras.

The numbers in this guide come from the Water Kiwoogi simulator model; real tanks can differ.