Hang-on setup

Measure before choosing a hang-on aquarium filter

A hang-on-back (HOB) filter can avoid a cabinet installation, but “hang-on” does not mean that every aquarium edge or wall position will work. The filter sits on the rim, draws water up through an intake and returns it from behind or above the tank. That creates several physical checks beyond the gallons printed beside its model name. This guide compares Fluval’s current AC20/30/50/70/110 and C2/C3/C4 lines with the separately listed earlier AquaClear 20/30/50/70/110. It does not cover every HOB brand or certify a stocking plan.

Start with the aquarium, not the capacity label

Fluval’s current US AC comparison assigns the five models these manufacturer-listed intervals: AC20, 5–20 US gallons; AC30, 10–30; AC50, 20–50; AC70, 40–70; and AC110, 60–110. The ranges overlap by design. At 25 gallons both AC30 and AC50 fall within their printed intervals; at 45 gallons the overlap includes AC50 and AC70. A finder should retain that ambiguity rather than pretend the larger label or a flow number alone identifies the “right” unit.

These are capacity labels, not independently verified stocking limits. They do not describe a particular aquarium’s animal size, feeding, waste, desired current or water-quality outcomes. None of the reviewed AC pages gives a model-specific high-bioload rating. If stock creates unusually high waste or has special current needs, treat the gallon match only as one input and seek husbandry guidance that addresses the species and system.

Current AC products are distinct from Fluval’s separately archived earlier-generation AquaClear items. The previous-generation US archive identifies AquaClear 20, 30, 50, 70 and 110 as A595F, A600F, A610F, A615F and A620F, respectively; the current AC line uses item numbers 19595, 19600, 19610, 19615 and 19620. The older pages print the same gallon intervals—5–20, 10–30, 20–50, 40–70 and 60–110—but do not supply the same installation, electrical and media details as the current series. When comparing a replacement, listing or media pack, check the exact generation and item identity rather than assuming that “20” or “50” proves interchangeability.

The current C2, C3 and C4 are a separate HOB line

Fluval’s US C-Series comparison lists C2, C3 and C4 as distinct powered clip-on filters rather than AC variants. Their printed capacity labels are upper-only: “up to” 30, 50 and 70 US gallons, respectively. In the reviewed sources there is no lower endpoint for any of the three, so the 30-gallon ceiling on C2 does not itself make it an exact 30-gallon interval match. The finder can identify them for the hang-on arrangement, but a typed aquarium volume remains unverified until a lower bound is established.

The models differ in footprint, circulation label and rim thickness: C2 is 6 × 4.7 × 6.7 inches and 119 US GPH with a one-inch maximum rim; C3 is 7 × 4.7 × 6.7 inches and 153 US GPH, also with a one-inch maximum; C4 is 8.3 × 6.1 × 8.5 inches and 264 US GPH with a 1 1/8-inch rim limit. All three list 5 W at 120 V / 60 Hz. Their five-stage path separates two mechanical, one chemical and two biological stages, including the Bio-Screen and C-Nodes trickle chamber. These are manufacturer-described specifications, not independent measures of installed output or stocking capacity.

The North American manual adds shared setup checks: keep the case level, fill it to prime, start with the flow control at minimum before adjusting after priming, and keep the waterline within 2.25 inches of the rim. It also includes a two-week media-cleaning recommendation. Measure lid and rear-wall clearance around the largest body and leave room for the staged media to be removed. The C-Series package is not a published litre basket volume, and no reviewed C model has explicit high-waste guidance. Do not convert five stages or an upper gallon label into either of those claims.

Earlier AquaClear models need their own checks

Fluval’s FAQ describes AC as a new generation with upgraded media, a self-leveling interface, re-engineered rotor, sound-dampening cover and new high-impact materials. It says the redesigned interface addresses rear-spill concerns from the earlier generation. These are manufacturer-described changes, not independent measurements or a prediction about a particular older filter. They explain why a legacy AquaClear 50 and current AC50 should not share every specification field. C2/C3/C4 are also their own current configurations, not earlier AquaClear sizes or aliases for the current AC line.

For the earlier A595F/A600F/A610F/A615F/A620F models, the reviewed US product pages name adjustable flow, a media basket and foam, carbon and BIOMAX; they do not give model-specific numeric flow, basket volume, electrical draw, dimensions or exact rim/wall/waterline limits. Do not transfer those numbers from the current AC pages. Measure the actual aquarium and obtain instructions matching the exact earlier unit. Fluval does confirm that current AC media—including the newer intake strainer sponge—fits previous-generation AquaClear filters. Its shared motor listing covers AquaClear 20/30/50/70 and AC20/30/50/70, not 110; verify each part’s exact fit rather than assuming universal interchangeability.

Measure the rim, back wall and waterline

The AC installation envelope varies by model. Fluval’s US FAQ gives a maximum rim thickness of 0.9 inch for AC20/30/50/70 and 1.4 inches for AC110. It specifies at least 2.75 inches of space between tank and wall for AC20/30/50, 4 inches for AC70 and 4.75 inches for AC110. The water level must stay no more than 5/8 inch below the output on AC20/30/50; AC70/110 allow a 1.5-inch gap. These are manufacturer limits, not a fit test of your tank.

Measure the assembled rim including braces, lid rails and trim. Also account for the filter’s body, access to its media chamber, any handle or lever movement, the light and nearby wiring. A tank that touches the wall may need to move forward; the clear space behind it must accommodate the unit, not merely its intake tube. A low waterline can leave the return louder or more exposed than intended, so check it against the model manual and keep the aquarium’s operating level in the stated envelope.

Body dimensions help screen a candidate but do not replace that clearance check. The published lengths are 5.1 inches for AC20, 6.1 for AC30, 7.2 for AC50, 8.5 for AC70 and 13.6 for AC110. Width and height also grow for the largest model: AC110 is listed at 6.9 × 9.2 inches compared with 4.4 × 6.4 inches on AC20/30/50 and 6.1 × 8.6 inches on AC70. Confirm which dimension the manufacturer calls length, width and height, then compare the installed orientation shown in the manual.

Keep flow terms in their own rows

The family table labels maximum flow as 100, 150, 200, 300 and 500 US gallons per hour for AC20 through AC110. It also describes adjustable flow and a refiltration path that can pass a portion of water through the chamber again at the lowest-flow setting. This is not an independent measurement of installed output through a particular intake, media load or aquarium. Nor is it a stated filter-circulation figure. Do not turn the highest GPH number into a promise of turnover, effective biological capacity or suitable current for a fish.

The comparison rows also show different power and physical size. AC20/30/50/70 list 7 W at 120 V/60 Hz and 5 W at 230–240 V/50 Hz; AC110 lists 14 W at both voltage rows. Check the regional electrical specification and nameplate of the actual unit, especially when a seller’s photo is not of the package being shipped. Manufacturer’s 120 V rating is not a substitute for safe outlet placement, drip loops or keeping electrical parts dry.

Use included media as a package fact, not a capacity rating

The current family comparison lists four supplied media stages for AC20, AC30, AC50 and AC70: one intake sponge, one Bio-Foam, one activated-carbon insert and one BIOMAX insert. AC110 shares the sponge and Bio-Foam but lists two carbon inserts and two BIOMAX inserts. The chamber arrangement and amount of media are not the same as a published litre measure; Fluval does not provide a basket-volume figure for these models in the reviewed comparison. Therefore a canister’s basket capacity cannot be carried over to an AC model, and media count is not a biological-capacity test.

Fluval describes a CycleGuard approach: replace only one of the three main insert types at a time rather than replacing all biological and chemical media together. The FAQ advises rinsing the intake sponge monthly and replacing it every two months, with more frequent service potentially needed as an aquarium gets dirtier. Follow the current manual for priming, media placement and safe cleaning; avoid translating a stated schedule into a guaranteed interval for a different bioload. Dirty media or an impeller can reduce flow, so a change in observed return is a reason to inspect and maintain the unit as directed.

Check the exact listing and model before purchase

Compare the five models on separately labelled facts: capacity interval, maximum-flow label, dimensions, power row, media counts and installation clearances. Do not fill an unknown field with an older AquaClear specification or an adjacent model’s value. The AC20/30/50/70 page describes a precision motor and stepped interface; those claims should not be extended to AC110 unless its own source supports them. A compatible optional surface-skimmer attachment is an accessory, not proof that it is in the filter box.

For a specific aquarium, measure the rim and available rear depth, choose the intended operating waterline, inspect cover/brace interference, plan access for routine sponge service, and verify the actual model and included inserts on the box. Only after these fit checks should the printed tank interval be compared with actual water volume. A model can pass one screen and fail another; this is a method for narrowing candidates, not a recommendation to buy a particular size.

Primary references: Fluval current AC Series comparison and included-media table, Fluval current C-Series comparison, North American C-Series manual, Fluval earlier-generation AquaClear archive, Earlier AquaClear 20 US item page, Earlier AquaClear 110 US item page, Current AC20/30/50/70 manual and Fluval AC Series / earlier-generation FAQ.