Filtration & circulation
Choosing a Filter for Your Actual Tank Volume and Fish
Compare published pump and circulation ratios across usable aquarium volumes, retain source conditions and choose filtration around fish, installation and maintenance.
1. Establish how much water the aquarium really holds
The volume printed on a tank’s box may describe an empty vessel filled differently from your aquarium. Substrate, wood, rocks and the space below the rim reduce the water held. Start with internal measurements and the working water level, then estimate displacement or record the water added during the initial fill. Our usable-volume calculator helps you document the assumptions rather than hiding them behind a precise-looking number.
For example, a tank advertised as 100 litres might contain an estimated 80 litres after setup. That example is not a standard deduction: another aquarium could lose much less or more. Use your estimate to understand circulation and some dosing instructions, while following each product’s actual dosing basis. Do not subtract so much displacement that you use a smaller filter despite a demanding livestock plan.
Include an external filter’s water only when you can estimate it sensibly, and keep display volume separate in your notes. A canister’s media basket volume is not its total water capacity. Recording “approximately 80 L in the display, external volume not measured” is more useful than pretending that the box label is an exact operating figure.
2. Describe the fish and feeding before setting a rating
Biological demand follows waste entering the system. Number of fish alone is a poor comparison: their adult size, diet and feeding quantity matter, as do uneaten food and accumulated organic debris. Research the intended species and appropriate aquarium dimensions before assuming extra filtration makes unsuitable stocking acceptable. Our equipment checklist puts filtration alongside the other setup requirements.
Write down the adult livestock plan, feeding routine and whether plants or hardscape obstruct the circulation path. A lightly fed aquarium and one with substantial waste production can need different maintenance and media arrangements even at the same litres. Manufacturer application ranges are a useful initial screen, but they do not model every possible combination of fish and food.
Do not interpret “up to 200 L” as a promise that a filter can manage any 200 L aquarium. Nor should a model’s larger rating persuade you to keep fish in too little swimming space. Filtration, aquarium dimensions and species suitability are connected decisions with different limits.
3. Identify which flow figure you are comparing
A maximum pump output is not necessarily the amount returned through a working filter. Media, hoses, height and fittings affect delivery, and debris changes it over time. Some manufacturers publish separate circulation figures; others publish a pump specification only. Compare like with like and record the source of each number.
| Illustrative calculation | Result | What it establishes |
|---|---|---|
| 400 L/h pump specification ÷ 80 L estimated display | 5 nominal tank volumes per hour | A ratio using the label, not a measured operating turnover |
| Fluval 207 reference labelled 120 V / 60 Hz: 780 L/h pump ÷ 80 L | 9.75 using pump output | Not equivalent to the stated filter-circulation calculation |
| Fluval 207 reference labelled 120 V / 60 Hz: 460 L/h circulation ÷ 80 L | 5.75 using circulation | Still not a measurement of this aquarium’s installed flow |
Choose the housing format alongside the rating. The internal-versus-external comparison connects swimming space, cabinet access and repeated servicing, so a promising flow figure does not lead to a filter you cannot maintain.
Compare published flow labels across usable volumes
We calculated how two differently labelled reference figures change across seven hypothetical display volumes. The current Fluval 207 UK web table explicitly labels its 780 L/h pump output and 460 L/h filter circulation entries 120 V / 60 Hz. That source condition matters: these are reference-label calculations, not a claim about the performance of a UK 230–240 V / 50 Hz installation.
The 07-series manual also distinguishes pump performance from filter circulation; its circulation footnote specifies intake and output hoses of equal length. Neither source establishes the actual return in your aquarium. Check the electrical variant and stated conditions of a flow figure before using it for a purchase decision.
| Assumed display water | 780 L/h pump reference | 460 L/h circulation reference |
|---|---|---|
| 60 L | 13 reference volumes/hour | 7.667 reference volumes/hour |
| 70 L | 11.143 reference volumes/hour | 6.571 reference volumes/hour |
| 80 L | 9.75 reference volumes/hour | 5.75 reference volumes/hour |
| 90 L | 8.667 reference volumes/hour | 5.111 reference volumes/hour |
| 100 L | 7.8 reference volumes/hour | 4.6 reference volumes/hour |
| 120 L | 6.5 reference volumes/hour | 3.833 reference volumes/hour |
| 200 L | 3.9 reference volumes/hour | 2.3 reference volumes/hour |
At 80 litres, choosing the pump entry gives 9.75 while choosing the circulation entry gives 5.75. The pump-based ratio is approximately 69.6% higher, calculated as (780 ÷ 460 − 1) × 100. That is a difference between two published reference numbers, not a measured percentage lost in every installation. Do not apply it as a universal reduction factor to other filters or to a dirty hose.
Changing the denominator does not change the pump
For either fixed reference figure, moving from an assumed 100 litres to 80 litres increases the ratio by 25%: 100 ÷ 80 − 1 = 0.25. For the circulation reference, that is 4.6 becoming 5.75. The equipment has not gained capacity; the denominator changed.
Volume uncertainty also creates a range. Using the same 460 L/h reference with 70–90 litres produces approximately 6.571–5.111 reference volumes per hour. The lower volume gives the higher ratio. Retain that uncertainty instead of choosing the smallest plausible denominator to make a filter look stronger. Our usable-volume scenarios explain where the litre estimate comes from.
Keep biological demand and circulation in the decision
A reference ratio does not describe how flow is distributed, whether fish can rest comfortably, how much waste enters the aquarium or how well established media process it. It also does not establish how many fish can be stocked. A return can short-circuit towards a nearby intake while another area remains poorly circulated.
Use the figures to compare the same kind of published specification under stated conditions, then check media, livestock, outlet options and installation. Keep cabinet and service requirements alongside the arithmetic in the internal-versus-external comparison. If a working filter's flow changes, follow the low-flow checks rather than treating a label-based ratio as a measurement of the fault.
Download fourteen ratios with the original labels, electrical basis and source links (CSV)
Values are rounded to three decimal places for comparison; the source figures and volume assumptions do not imply that precision in actual aquarium flow. The download retains calculation results to six decimal places for reproduction.
4. Check internal fit and return control
For an internal filter, measure water depth and the room for its bracket, intake and media removal. The Fluval U2 is one example with several return arrangements and an up-to-110 L maker label. Its integrated spray bar and alternative outlets can help distribute movement, but you must still check the actual fish and maintenance access. A return that continually pushes a fish away from food is a layout problem to resolve, not evidence that the filter is working especially well.
Adjustable internal filtration
Fluval U2 internal filter
A three-stage internal filter with a flip-top lid and a choice of outlets. It is worth considering when you want accessible media and control over how the return water moves.
- Up to 110 L maker rating
- 400 L/h rated output
- Three-way flow control
An aquarium where the unit fits without crowding the swimming space and the flow can be set for the fish.
- The upper litre rating is not a stocking guarantee.
- Takes space inside the tank; check room to lift out the media.
Try an appropriate outlet direction before restricting anything. Water should move through useful parts of the aquarium, not race between an intake and a nearby return while debris collects elsewhere. If an internal housing consumes too much working space, compare internal and external formats rather than buying the next strongest internal by default.
5. Check canister media space and the cabinet together
The Fluval 207 illustrates why a canister’s published capacity and practical installation need separate checks. The body measures 19 × 18 × 42 cm, and there is additional space to allow for hoses and servicing. The maker lists a 1.8 L basket volume, but filling every gap indiscriminately can obstruct the intended water path. Use compatible media in the instructed order.
Accessible baskets in a narrower cabinet
Fluval 207 external canister filter
A canister with removable media baskets, a primer and AquaStop control. Its published pump output and filter-circulation figures describe different operating conditions.
- 19 × 18 × 42 cm body
- 1.8 L published basket volume
- 10 W at 230–240 V / 50 Hz
A setup that suits the maker’s 60–220 L range, with room for the body, hoses and service movement.
- Allow extra height for hoses and removal; 42 cm is not the whole service envelope.
- Flow regulation has a manual-defined limit; do not close the operating valve beyond it.
- The current web flow table labels its figures 120 V / 60 Hz; check the data for your electrical variant.
Measure the cabinet opening, the location of shelves and the movement needed to disconnect the hose assembly. Consider whether you can lift the filled filter safely and inspect its connections after maintenance. Follow the exact height and priming instructions. If all these requirements cannot be met, a theoretical increase in media volume is not a practical improvement.
6. Confirm the choice with water results and observation
A new filter must mature before it can be trusted with the planned waste load. Follow a fishless cycling process before stocking a new aquarium. When changing an established setup, preserve established filtration and monitor ammonia and nitrite rather than discarding the old media because the new pump is larger.
Once running, look for suitable surface movement, accessible resting areas and waste collecting in persistent dead spots. Observe fish at feeding time as well as when the tank is quiet. Adjust the return within the manufacturer’s permitted range and recheck both the surface and the far end. A directional change or compatible spray bar may help more than an increase in pump size.
If flow falls, compare it with the filter’s normal clean condition. Check water level, the intake, media and hose route before blaming capacity. Our low-flow troubleshooting guide explains the order. Keep a simple record of the model, outlet setting, media arrangement and water results so that future changes have a clear starting point.
The final choice should pass four checks: appropriate biological capacity, comfortable and useful circulation, a physically suitable installation, and maintenance you can perform without avoiding it. Buy around those requirements rather than choosing the highest number on the shelf.
Common questions
Should I size a filter for gross or net aquarium volume?
Use usable volume to understand the water actually present, but also account for adult livestock, feeding and the maker’s application range. A lower net volume is not permission to ignore a demanding waste load or inadequate tank dimensions.
How many times per hour should my aquarium turn over?
There is no universal target suitable for all fish, layouts and filter types. Identify whether the published figure is pump output or circulation, then assess biological capacity, water results and movement throughout your own aquarium.
Does a larger filter allow more fish?
A filter cannot create swimming space or correct incompatible livestock. Appropriate stocking depends on species, adult size, aquarium dimensions and water conditions as well as waste processing. Review the livestock plan before using equipment size to justify additions.
How can I estimate displacement without dismantling the tank?
Use internal dimensions and the working water level, then make a transparent estimate for substrate and decor. Keep a range if necessary. At a future full setup, recording water added provides a better starting reference.
Does more biological media fix ammonia immediately?
New dry media is not mature. It provides potential surface that must become established under suitable conditions. Investigate ammonia promptly and preserve established filtration; simply adding a bag of rings does not prove the aquarium is processing its waste safely.
What should I check after changing the outlet setting?
Check movement at the surface and behind decor, observe feeding and resting behaviour, and confirm the pump operates within its permitted range. Continue normal water-quality monitoring because a comfortable-looking return alone cannot demonstrate biological performance.