Equipment sizing
Aquarium heater size guide
The dial lies. Room temperature, open tops, and the species target decide whether a wattage holds. Check with a probe — not the sticker on the heater.
What actually drives heater size
- Tank volume — more water needs more watts for the same lift.
- Room temperature — a cold garage in January is a bigger fight than a warm living room.
- Target water temperature — discus near 28–30°C need more lift than a community at 24°C.
- Open tops and big surface area — heat leaves faster.
Mid-size tank, warm room, mid-20s °C water: start near 1 W per litre. Nanos lose heat faster and often need more per litre — use the table. Cold rooms, open tops, hotter targets, and thin lids all push wattage up.
Wattage by tank size
Warm room: rarely below about 18°C. Cold room: stretches near or below 15°C. In between, sit between the columns and re-check at night in winter. From about 200L up, two heaters that add up to the band usually beat one big unit.
| Tank | Warm room (~>18°C) | Cold room (~<15°C) |
|---|---|---|
| 30L · 8 US gal | 50–75W | 75–100W |
| 40L · 11 US gal | 50–75W | 75–100W |
| 60L · 16 US gal | 75–100W | 100–150W |
| 80L · 21 US gal | 100–125W | 125–150W |
| 100L · 26 US gal | 100–150W | 150–200W |
| 120L · 32 US gal | 150–200W | 200–250W |
| 200L · 53 US gal | 200–250W (or two heaters totaling this) | 250–300W (or two heaters totaling this) |
| 300L · 79 US gal | 250–350W (prefer two totaling this) | 350–400W (prefer two totaling this) |
Stocking, room temperature, tank shape, and how often you clean all move the answer. The tables here are shortlist bands — check them with a probe thermometer, your eyes on flow, and liquid tests.
Warm vs cold midpoints
Same bands as the table — midpoints for a quick glance.
Two smaller heaters vs one large heater
On bigger tropical tanks, split the column’s watts across two units at opposite ends. One dies off → the other slows a cold crash. One sticks on → the second thermostat does nothing useful; it just switches off while the failed unit keeps heating. Split wattage only limits how hard one failure can cook the tank compared with one oversized heater stuck on.
Keep a probe. For discus and other heat-sensitive fish, add a controller with a high-temp cut-out. A spare heater in the drawer is cheaper than a winter crash. Check targets on fish profiles and temperature overlap in the tank builder.
Open tops and nano swings
Open tops bleed heat. Tall skinny tanks stratify if flow is weak. Nanos swing hard every time the heater cycles. Treat the table as a floor. Check at night when the room drops — one daytime reading is not enough.
Placement
Sit the heater in flow from the filter return or a circulation pump so warm water mixes. Keep the tube fully submerged to the maker’s minimum waterline. Do not bury glass heaters in deep substrate where heat can trap.
Thermostat accuracy
Dials drift. After seasonal weather changes, compare the heater setting to a digital probe at the far end of the tank. Stick-on liquid-crystal strips read glass temperature and routinely mislead.
Probe thermometer
A heater without a separate thermometer is guessing. Check morning and night when rooms swing. Species ranges on fish profiles beat a single “community” number.
Guards and large fish
Large cichlids and some catfish can crack unprotected glass heaters. A heater guard or careful hardscape placement reduces contact. Follow the manufacturer’s immersion and guard guidance for your model.
Failures worth planning for
- Stuck on. Cooks fish fast. Split wattage slows the climb; it does not stop it. A controller with a max-temp cut-out plus a probe is the real safety net.
- Stuck off in winter. Tropicals chill. Keep a spare heater if the stock needs heat.
- Running dry. Do not power a glass heater in air — it can crack when you dunk it again. Unplug before water changes that expose the tube.
- No flow around the heater. Hot pocket near the tube, cool water at the far end — especially in small tanks.
Follow the maker’s instructions and local electrical codes. Use aquarium-rated gear. Keep cords and sockets dry.
Match heat to the species list
Pick a watt band here, then confirm every fish overlaps on temperature in the tank builder.
Related: filter guide · tank setup · 60L hub · 100L hub · 200L hub
Sources and limits
Stocking, room temperature, tank shape, and how often you clean all move the answer. The tables here are shortlist bands — check them with a probe thermometer, your eyes on flow, and liquid tests.
- Aquarium Co-Op — The Nitrogen Cycle — Clear ammonia → nitrite → nitrate walkthrough for cycling before livestock.
- Seriously Fish — species care profiles — Temperature and flow notes when you leave the generic table for a real stock list.
Heater sizing — quick answers
- What watt heater do I need?
- Mid-size tank, warm room, mid-20s °C water: start near 1 W per litre, then adjust. Nanos lose heat faster and often need more per litre. Cold rooms, open tops, and hotter targets push wattage up. The table shortlists; a probe decides.
- Should I use two heaters?
- On larger tropical tanks, yes — two units that add up to the band. If one dies off, the other slows a cold crash. A second thermostat will not cool a stuck-on heater. Split wattage only limits how hard one failure can cook the tank. Still keep a probe (and a controller for discus-level heat).
- Where should I place an aquarium heater?
- In flow from the filter or a pump so heat mixes. Submerge to the maker’s mark. Do not bury a glass tube in deep sand where heat traps.
- Are stick-on strip thermometers enough?
- They read the glass. Use a digital probe away from the heater outlet for the water the fish actually feel.