What goes wrong if the pump is undersized
Undersized air means sponge filters slow down — water stops moving through the media, biofiltration drops without warning, ammonia climbs days later. With air-stone-only setups, the visible failure is fish at the surface gasping at first light, when oxygen is at its daily low. Both are preventable by sizing the pump to the tank, not the box copy.
The hobby anchor is roughly 0.033 liters of air per minute per gallonof freshwater for an air-stone — the calculator's starting point. It's not a peer-reviewed constant; it's an industry rule of thumb that happens to line up cleanly with how manufacturers map pump SKUs to tank-gallon ratings on the box.
The rule is for an air-stone driving surface agitation— bubbles breaking the surface, gas exchange, the visual. It's not a biological-oxygen-demand calculation. Stocking density, plant load, temperature, and the presence of HOB/canister filters (which already do most of the surface agitation work) all shift what your tank actually needs. If your filter return is breaking the surface well, you may not need a pump at all — see the last section.
The Equipment cluster on the calculators hub covers the three sizing decisions you make once when the tank is empty — heater wattage, filter flow, and air-pump output.
Sponge filters change the math
A sponge filter doesn't just want bubbles — it wants enough air to drive water up through the sponge media. The forum-aggregated standard across multiple sources is roughly 1.0 L/min per sponge filter, bumping to about 1.5 L/min once the tank is taller than ~18 inches. Past that depth the air also has to fight head pressure to reach the bottom of the uplift tube.
The calculator picks whichever number is bigger — the per-gallon air-stone baseline or the per-sponge demand — so a 10-gallon shrimp tank with one sponge filter ends up driven by the sponge (1.0 L/min), not by the per-gallon math (0.33 L/min). It tells you which one is in the driver's seat in the result panel.
Don't undersize the pump for sponge use. A too-quiet pump means the sponge slowly gunks up, water flow drops, and biofiltration tapers off without an obvious symptom for weeks. The next time the tank is stressed (skipped water change, fish addition, heat wave) ammonia climbs because the colony was already running thin. Pick one tier of headroom over the calculator's number — the cost delta is a few dollars; the failure mode is a tank crash.
Why depth gets a flag, not a number
Depth changes the pressure the pump has to push against — the head height of the water column — not directly the L/min you need. Most consumer air pumps are designed for tanks up to ~18 inches deep. Past that, the pump can be rated for 2 L/min and still struggle to push bubbles to the bottom of a 24-inch tank. The honest answer is that this spec — head pressure — is something consumer brands rarely publish.
The signal that depth matters is that manufacturers ship distinct depth-rated SKUs. Tetra's “Whisper Deep Water” line exists specifically for tanks deeper than standard. If your tank is over 18 inches, the calculator surfaces a flag and you should look for a depth-rated model rather than just the highest L/min number on the shelf.
Tank-type adjustments
The freshwater baseline is the default. Salt and planted both move off it for reasons that have nothing to do with the fish — the water itself behaves differently — so don't skip these even if your stocking looks similar across tank types.
- Planted tanks — about 80% of the freshwater baseline. Surface agitation outgasses dissolved CO₂, which is usually the limiting nutrient in a planted setup. Run the air at night when CO₂ is off and plants stop producing oxygen.
- Saltwater / reef — about 120% of freshwater. At tropical temperatures, salinity reduces dissolved-oxygen saturation by roughly 20%, so the same fish in salt need more aeration to hit the same DO ppm.
- Coldwater / goldfish — counts as freshwater for this calculator. Goldfish are heavy oxygen consumers but cold water also holds more dissolved O₂, and the two effects roughly cancel for sizing purposes.
Reference SKUs by L/min
From manufacturer spec pages — listed as a reference table, not an endorsement. Use these to sanity-check the calculator's number against what's actually on the shelf. The opinion baked into the table: don't chase the highest L/min on the shelf, and don't buy at the floor of the calculator's number either. Match the number, then jump one tier — pumps degrade, diaphragms tire, and an extra 1 L/min of headroom buys years.
Values prefixed with ~ are aggregated from retailer listings rather than verified directly from the manufacturer page at last check (May 2026). See the sources block at the bottom for which rows are primary-sourced vs aggregated.
| Pump | L/min | L/h | GPH |
|---|---|---|---|
| Tetra Whisper 10 | ~0.5 | ~30 | ~8 |
| Tetra Whisper 20 | ~1.0 | ~60 | ~16 |
| Aquarium Co-Op USB Nano (single) | ~1.1 | ~66 | ~17 |
| Tetra Whisper 40 | ~1.3 | ~78 | ~21 |
| Eheim Air 100 | 1.67 | 100 | ~26 |
| Tetra Whisper 60 | ~2.6 | ~156 | ~41 |
| Tetra Whisper 100 | ~3.3 | ~200 | ~52 |
| Eheim Air 200 | 3.33 | 200 | ~53 |
| Eheim Air 400 | 6.67 | 400 | ~106 |
| Hygger Extra Quiet 6W | 12.0 | 720 | ~190 |
1 L/min = 60 L/h = ~15.85 GPH. Manufacturers publish in different units — Eheim in L/h, Tetra in L/min, US retailer listings often in GPH — so the same pump can look bigger or smaller depending on which spec sheet you're reading.
Do you actually need an air pump?
Many tanks running an HOB or canister filter don't. The filter return already breaks the surface and that's usually enough gas exchange for typical stocking. An air pump becomes more important when you're running a sponge or undergravel filter (where air IS the drive), keeping a heavily-stocked tank, running warm water (lower DO solubility), or dosing CO₂ during the day and need night-time aeration to recover.
Related tools
- Tank volume calculator — get accurate gallons first.
- Filter flow rate calculator — sister sizing tool for the main filter.
- Heater wattage calculator — the third Equipment-cluster sizing decision.
- Filter types explained — pick filter mode (sponge vs HOB vs canister) before sizing the pump.
Sources verified May 2026.The 0.033 L/min/gal freshwater baseline is an aquarium-industry rule of thumb that lines up with how manufacturers map pump SKUs to gallon ratings on the box, not a peer-reviewed constant. The 1.0 L/min-per-sponge figure is synthesized from established hobby references and consistent with Aquarium Co-Op's public guidance pairing a single-outlet nano pump with one sponge filter. The 80% planted and 120% reef multipliers reflect the standard freshwater-vs-saline DO solubility delta at tropical temperatures.
Manufacturer specs in the SKU table:
- Eheim Air 100 / 200 / 400 — verified from eheim.com: 100 L/h single outlet, 200 L/h (2×100), 400 L/h (2×200).
- Tetra Whisper series and Tetra Whisper Deep Water — manufacturer page (tetra-fish.com) was not directly reachable at verification time; L/min values in the table are aggregated from retailer listings (Petsmart, manuals.plus, petscience.ca) that quote Tetra's published specs. Treat the Tetra rows as approximate.
- Aquarium Co-Op USB Nano — Cory's public note that the unit “is powerful enough to run a sponge filter in Cory's 800-gallon aquarium, which is 40 inches in height” is from the Aquarium Co-Op product page. Specific L/min ratings are not currently published on that page; the 1.1 / 1.6 L/min figures in the table reflect the previously-published single- and dual-outlet specs and are treated as approximate.
- Hygger Extra Quiet 6W and 10W — published L/min and GPH ratings on hygger-online.com.
- Fluval Q1 / Q2 — Fluval publishes ratings as tank-volume ranges (170–300 L for Q1, 190–600 L for Q2), not raw L/min, in the published Fluval Q-Pump manual.
If a Tetra or Aquarium Co-Op spec moves materially after a future verification pass, update both the table here and COMPETITOR_INTEL.md in the FishTankMath repo at the same time.