TOOL 12
Flow & Turnover
Calculator.
Find the pump or filter flow you need for a healthy turnover rate, then check what a real-world pump delivers after head loss and plumbing restrictions.
Required flow = tank volume × turnover per hour. Real pumps deliver 60–80% of their rated figure once head height, bends and media are accounted for.
ABOUT THIS TOOL
Flow & Turnover
Calculator.
Flow rate is one of the most under-appreciated factors in a successful aquarium. It drives the filter, distributes oxygen and heat, keeps debris suspended so mechanical filtration can capture it, and — for many fish — is a direct welfare issue. A tank with too little flow becomes stagnant and oxygen-poor; too much turns into a washing machine that exhausts its inhabitants.
The usual way to specify flow is turnover: how many times per hour the entire tank volume passes through the filtration system. Community tanks typically target 4–6 times per hour, planted tanks a little higher, and riverine or high-energy setups much more. Multiplying your real water volume by your target turnover gives the flow you need at the outlet.
The catch is that a pump's rated flow is measured with no resistance. Real installations add pumping head from the height difference, friction from pipe length, and loss through bends and filter media, so a pump rarely delivers its nameplate figure. This calculator also estimates delivered flow so you can size a pump for what it will actually do, not what the box claims.
HOW TO USE
Using the
Calculator.
- Enter your water volumeUse your true water volume from the Tank Volume Calculator, in litres or US gallons.
- Set a turnover targetChoose how many times per hour you want the volume cycled. Four to six suits most community tanks; higher for planted or riverine species.
- Read the required flowThe primary result shows the flow you need at the outlet, in litres per hour and US gallons per hour.
- Enter your pump ratingAdd the rated flow from your filter or pump, and its height above the tank or sump water line.
- Check delivered flowThe calculator reduces the rated figure for head and restrictions to estimate what reaches the tank.
- Compare and adjustIf delivered flow is below the requirement, size up, reduce head, straighten plumbing, or add a circulation pump.
FAQ
Flow
Questions.
What turnover rate should I aim for?
Most community aquariums do well at 4–6 turnovers per hour. Planted tanks often run 6–8, and biotope tanks for riverine fish such as hillstream loaches can run 10 or more. Very low-flow species like bettas and some gouramis prefer less.
Why does my pump not match its rating?
Pump ratings are measured at zero head with no plumbing. Once you add the height it must lift water, the friction of pipe runs, and the resistance of filter media, real delivery drops — often to 60–80% of the rated figure, or less.
How does head loss work?
Every centimetre the pump must lift water costs flow, and each bend, narrow section, or block of media adds friction loss. A long return line with several elbows can cut output dramatically compared with a short, straight run.
Can I have too much flow?
Yes. Excessive flow stresses fish that evolved in calm water, uproots plants, and can push food past fish before they eat. Aim for strong circulation with calm refuge zones rather than uniform blast.
Should the filter turnover include the sump?
For a sump system, base turnover on the display tank volume, but remember the pump must also lift water from the sump. The head difference between sump water level and display outlet is the height that matters.
Is filtration flow the same as circulation?
Not always. You can have high circulation from powerheads and low filter throughput. For water clarity and biological filtration, ensure the filter itself turns over the tank adequately, then add circulation if specific species need more.