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Start with the tank, then check the filter
Choosing by the biggest gallon number is quick, but it skips several ways a filter can fail to fit the real aquarium. A useful comparison starts with volume and then checks stocking, physical installation, media plan and the routine you will actually maintain.
1. Measure water volume honestly
Use the tank's approximate operating water volume in US gallons, not its outside dimensions or an empty glass capacity. If the tank is a nonstandard shape or has a deep substrate and hardscape, the water volume may differ from the nominal tank size. Manufacturer aquarium-size ranges can make a first shortlist, but they are not a stocking chart. Our finder matches both endpoints of a model's stated interval; it does not assert that every fish combination within that interval is safe.
Read the exact edition's printed unit range. Fluval's current 07 materials pair US gallons and litres, but some displayed pairs are not direct unit conversions. When the numbers matter, use the US gallon entry for the US model and check the manual or seller listing rather than silently recalculating one range into the other.
2. Treat livestock load as a separate question
Two tanks with equal water volume can ask different things of filtration. Fish size, stocking density, feeding, waste, plants, feeding leftovers and husbandry change the biological load and service frequency. Fluval's 07 FAQ names growing or newly added fish and increased waste as reasons filtration load may rise. Its documents do not give the 107, 207, 307 or 407 a fish-count or high-bioload rating. If an unusual or heavy load is central to the decision, seek species- and husbandry-specific guidance instead of treating a larger pump-output number as a guarantee.
The reviewed FX2, FX4 and FX6 pages likewise provide maximum tank-volume statements, not fish-count or high-waste ratings. A volume ceiling should not be treated as proof of fit for a particular stocking plan.
3. Choose a type that can be installed
External canisters keep the filter body outside the aquarium but need a safe below-tank location, water-carrying hoses and service access. For Fluval's 07 Series, the manual says the canister must always sit upright below the tank. Its specific maximum water-column allowance ranges from 4.5 feet for the 107 and 207, to 5.75 feet for the 307 and 7 feet for the 407. Measure the real distance from the canister base to the tank waterline, and check the allowed hose path and clearance.
The FX family is also gravity-fed and must never be positioned above the aquarium water level; Fluval recommends that it sit entirely beneath the tank for best performance. Its placement diagram and hose limits differ from the 07 values, so check the FX manual rather than transferring a neighboring canister's installation numbers.
Internal and hang-on arrangements involve different space, rim and service questions. A small room, shallow tank, tight cabinet or open-top aquascape can change which design makes sense. The relevant issue is not which label sounds more advanced; it is whether the exact unit can be mounted and maintained correctly.
4. Compare the numbers that represent the same thing
Do not compare pump output, manufacturer's filter-circulation figure, basket volume, chamber volume and canister volume as if they were interchangeable. Fluval reports two different GPH figures for each 07 canister and notes a measurement condition for circulation. It lists media basket and total vessel volumes separately. For instance, a 407's 4.2-litre basket is not the same as its 6.0-litre chamber or 9.2-litre canister. A higher value in one column does not prove better results in a particular tank.
5. Make maintenance part of the choice
Ask how you will reach the intake, impeller, gasket, media and hoses; how much room is available for a wet service task; which supplies the package includes; and whether you are willing to follow its schedule. Fluval's manual lists recurring care across components and warns against replacing all media at once. A model that fits on paper but is awkward to service may not fit your routine.
Use this order: actual water volume → livestock-load guidance → available installation position → media needs → maintenance routine → exact package check. The partial comparison set has 22 individually researched Fluval filter entries: 17 current models (seven external canisters, five U/Nano internal filters and five AC hang-on filters) plus five earlier-generation AquaClear hang-on models. It is not every aquarium filter. FX, U1 and Nano sources publish ceilings without lower volume bounds, so those models remain unconfirmed for a volume-only finder match. Both current AC and earlier AquaClear models have two-ended capacity intervals, but older models retain important specification gaps; no reviewed model has a verified high-load rating. Earlier models are opt-in and remain separate from current AC namesakes.
Primary references: Fluval 07 Series model page, 07 Series manual, 07 Series FAQ, FX Series model table, FX Series manual, AC Series and AC Series FAQ.