Aquarium Filter Flow Rate Calculator
Filters are sized by turnover: how many times per hour the full tank volume passes through the filter. This tool converts your tank size and stocking style into a target flow rate — and then corrects for the fact that a filter’s box rating is measured with no media, no hoses, and no head height.
Find your target flow rate
How turnover targets work
The widely used guideline for freshwater tanks is a turnover of 4–10 tank volumes per hour: around 4–6× for lightly stocked planted tanks, 6–8× for a typical community, and 8–10× for messy, high-waste fish such as goldfish, large cichlids and fancy plecos. The multiplier reflects how quickly waste is produced and how much water movement the fish tolerate.
The rated-flow trap
Manufacturers measure flow with an empty canister, no media, zero head height, and short or no hoses. In a real installation — media packed in, hoses run to a cabinet, water lifted a metre from sump or canister to rim — delivered flow commonly drops by 30–50%. That is why this tool tells you both the real flow you need and the (larger) box rating to shop for. A “1000 L/h” canister delivering 600 L/h is normal, not defective.
Flow is not the whole story
Turnover measures water movement through the filter, but biological filtration capacity depends just as much on media volume and surface area. A large, slow canister can out-filter a small, fast hang-on-back. Think of turnover as the circulation requirement and media volume as the processing requirement — you want both adequate. And remember some fish (bettas, gouramis, fancy goldfish) dislike strong currents: reach high turnover with spray bars or two outlets rather than one fire-hose jet.
Common mistakes
- Buying exactly the rated flow you need. You’ll get 60% of it. Buy up.
- Chasing turnover with a tiny media basket. Internal filters with high pumps but a sponge the size of a matchbox move water without processing it.
- Never cleaning the filter. A clogged filter can lose half its flow. Rinse mechanical media in tank water (never chlorinated tap water — it kills the bacteria) every few weeks.
- One outlet, dead spots everywhere. Debris piling in corners signals poor circulation even when turnover is theoretically high.
Frequently asked questions
Can I have too much flow?
Yes, for the fish rather than the filtration. If fish are constantly fighting the current, hiding in dead spots, or struggling to feed, spread the same flow over more outlets or throttle the pump. Long-finned and slow-bodied species are most affected.
Do turnover rules apply to sumps?
The biological logic is the same, but sump systems often run 5–10× through the sump for other reasons (surface skimming, oxygenation). Check your overflow’s rated capacity and see the pipe flow calculator for safe drain sizing.
Is a higher turnover a substitute for water changes?
No. Filters convert ammonia to nitrate but nothing removes nitrate as reliably as a water change. High flow with poor maintenance just produces very well-circulated nitrate. See the water change calculator.
Should I run two filters?
On tanks above ~150–200 L it is genuinely better: redundancy when one fails or clogs, staggered cleaning so you never wipe out your whole bacterial colony at once, and better circulation from opposite ends.