Water flow
Filter turnover is usually quoted as tank volumes per hour, and the manufacturer's figure is measured with no media and no head height. Real flow through a loaded filter is substantially lower, which is why a 'four times turnover' filter often behaves like a two-times one after a month.
In one sentence
Flow moves oxygen and nutrients through the tank, keeps waste in suspension where the filter can collect it, and is itself a habitat requirement — hillstream species need it and blackwater species need the absence of it.
How to read a measurement
Bands are guidance for interpreting a reading, not thresholds with sharp edges. Sensitivity varies by species and by how long the exposure lasts.
| Band | Range | Verdict | What it means |
|---|---|---|---|
| Still to gentle | ≤ 2 | acceptable | Suits blackwater and slow-water species — bettas, many anabantoids, and most floating-plant setups. Requires attention to surface movement, because low flow and poor gas exchange travel together. |
| General community | 3–5 | target | The band most community aquaria are run at. Turnover is a crude proxy: where the flow goes matters more than how much of it there is, and a tank can have a fast end and a slack end at any turnover figure. |
| High flow | ≥ 8 | watch | Appropriate for hillstream and riverine species that need it, and stressful for anything that does not. A fish that cannot hold position is spending energy it should be putting into growth. |
Matching flow to the animals
| Preference | Typical turnover | Examples |
|---|---|---|
| Still | 2–3× | Betta, sparkling gourami, clown killifish |
| Low | 3–5× | Most tetras, rasboras, dwarf cichlids |
| Moderate | 5–8× | Corydoras, danios, rainbowfish |
| High | 10× and above | Hillstream loaches, Denison's barb, bamboo shrimp |
Dead spots
Turnover is an average. A tank can have adequate turnover and still have corners where nothing moves, detritus accumulates and the substrate goes anaerobic. Aim the outflow so that water reaches the far end and returns rather than short-circuiting back to the intake.
How it is measured
- Pump rated output divided by tank volume, which overstates real turnover — head height, media and pipework all reduce it, frequently by a third or more
- Observing where detritus settles, which shows dead spots that a turnover figure cannot
Typical aquarium range
- Typical
- 4–6 × tank volume/h