Compare styles
External canister, internal and hang-on aquarium filters
Filter style is a physical arrangement, not a quality ranking. Before comparing pump numbers or media baskets, decide where the unit can sit, how it connects to the aquarium and how you will maintain it. The current reviewed subset covers Fluval 07 and FX external canisters, U Series and Nano internal filters, and five current AC hang-on models. Previous-generation AquaClear models, OASE, other manufacturers and mixed accessory rows remain in the wider catalogue research queue.
External canisters
An external canister sits outside the aquarium and routes water through hoses to and from the tank. Fluval's 07 Series is a concrete example: its manual calls it a gravity-fed external filter, requires it to stand upright below the aquarium and gives a model-specific maximum distance from its base to the waterline. Hose routing, air at the intake, priming and water-safe service access therefore matter alongside the body dimensions. The external position may free interior space, but it adds hose and cabinet requirements that should be checked before purchase.
Do not read a maximum water-column measurement as a general promise that a canister can go anywhere. The manual's below-tank rule remains in force. The 07 models also have different maximum distances, so a 107's limit should not be assigned to a 407, or vice versa.
The FX family has its own placement drawing and hose-length rules. Its manual says never to install above aquarium water level and recommends positioning the filter entirely beneath the tank for best performance. The FX4 and FX6 add a utility drain valve for canister drainage and water changes; the FX2 has no such valve. These differences affect installation and service rather than making one model universally better.
Internal filters
An internal filter is installed inside the aquarium, so available water depth, interior footprint, intake placement, current pattern and the ability to remove it for care are central checks. Fluval’s U1/U2/U3/U4 family supports vertical or horizontal positioning and offers a top outlet, integrated spray bar or bottom outlet; those are selectable routes, not simultaneous settings. The A455 Nano is a different internal model with suction-cup attachment and an adjustable outlet/spray bar. It has no published dimensions in the reviewed US listing. Review the exact model’s water depth, bracket, media, electrical details and service clearance before assuming it works in a shallow setup.
Because equipment occupies tank space, consider how it affects the aquascape, animals and maintenance access. A compact exterior measurement alone does not tell you whether the unit will sit clear of substrate, plants or a sheltered fish area. Check the manufacturer's placement instructions and observe whether the intended flow arrangement suits the aquarium.
Hang-on or clip-on filters
A hang-on filter uses the aquarium rim, so the tank needs a compatible edge, open clearance behind it, enough room for return flow and safe access to the media chamber. Fluval's current AC series publishes model-specific limits: AC20/30/50 allow up to 0.9-inch rim thickness, need a 2.75-inch rear-wall gap and keep the water within 5/8 inch of the output; AC70 shares the rim limit but needs a 4-inch wall gap and allows a 1.5-inch waterline gap; AC110 changes all three to a 1.4-inch rim, 4.75-inch wall gap and 1.5-inch waterline gap. Those are fit checks, not results of a mounting trial. Measure the actual aquarium, lid, light and service space before comparing gallon labels.
Compare arrangements before comparing size
Ask these questions in order: Is there a below-tank space and hose route? Is there a safe place inside the tank without consuming needed habitat? Does the aquarium rim accept a hang-on unit? Can the exact model be installed and serviced with the cover, lid and surrounding equipment in place? Can it be powered and primed as instructed? Those answers can rule out an otherwise plausible volume range.
Then compare exact specifications that have the same meaning. Pump output is not the same as filter circulation, and neither is identical to the AC Series' maximum-flow figure. Total canister volume is not the same as media-basket volume. The current AC family lists one each of intake sponge, Bio-Foam, carbon and BIOMAX for AC20/30/50/70, but two carbon and two BIOMAX for AC110; it does not publish a basket-litre figure. A style should not be selected from a single tank-size range or an unsupported fish-load claim.
Primary references: Fluval 07 external-canister family, Fluval FX external-canister family, FX Series manual, Fluval U Series internal filters, Fluval AC Series comparison, AC20/30/50/70 manual and AC Series FAQ. Image access and non-Fluval inventory remain incomplete.