Fit and capacity

Choose an aquarium filter without treating gallons as a load rating

Tank volume is the easiest dimension to see on a box, but a good buying decision also respects the hidden variables: livestock, waste, installation geometry, media choice and care time. The individually researched Fluval subset now contains 22 filter records: 17 current 07, FX, U, Nano and AC models plus five earlier-generation AquaClear clip-on models. Capacity wording differs: U2/U3/U4, current AC20–AC110 and earlier AquaClear 20–110 have two-ended intervals, while U1/Nano and FX sources provide ceilings without a lower bound. This remains a partial manufacturer and market inventory, not a category-complete list.

Use the rating as a boundary, not a promise

Fluval lists the 107 at 10–30 US gallons, the 207 at 20–45, the 307 at 40–70 and the 407 at 50–100. Those manufacturer intervals overlap: the 107 and 207 both include 20–30 gallons; the 207 and 307 both include 40–45; and the 307 and 407 both include 50–70. Overlap is a reason to examine other fit details, not an endorsement of one model as universally better. At 45 gallons, for example, both the 207 and 307 fall inside their listed intervals, though their media-basket sizes differ.

The FX2, FX4 and FX6 use different wording: Fluval lists them for aquariums up to 175, 250 and 400 US gallons respectively. Those are ceilings, not two-ended intervals, and the reviewed pages do not state a lower gallon boundary. The finder therefore does not promote an upper-only FX capacity claim to a fully confirmed volume match. Compare its maximum claim with the exact model page, then verify aquarium, livestock and installation needs independently.

The earlier AquaClear 20/30/50/70/110 product pages repeat the current AC family's printed gallon intervals but represent different item codes and designs. Fluval describes the later AC updates—media, self-leveling interface, rotor, cover and materials—on its FAQ. The older US pages do not establish equivalent current flow figures, numeric installation clearances, wattage or basket volume. Keep those unknown rather than importing a current AC value simply because the gallon interval is the same.

Aquael's current US external-filter table offers a separate example of how size ranges can overlap without making one model universally preferable. The ULTRA FILTER 900 spans 15.8–52.8 US gallons; its 1200 spans 42.3–79.2, and the 1400 68.6–158.5. Three distinct ULTRAMAX table rows overlap them at different points: 1000 item 123041 is 25–80 gallons, 1500 item 123040 is 65–120, and 2000 item 123039 is 105–185. These are manufacturer interval screens, not stocking recommendations; a 50-gallon tank falls within several Aquael intervals and still needs a species-appropriate load plan and an installation check.

All six Aquael canisters require a below-waterline position under their English manual; a reported tank range does not override that room requirement. Each Aquael tray is listed at 1.9 L, but basket count and total-bucket size differ. The ULTRAMAX buckets rise 11/13.5/16 L across 1000/1500/2000 while each tray remains 1.9 L; neither the whole canister nor its separate approximately 0.6-L prefilter is the same measurement. The US table does not establish the current ULTRAMAX voltage/frequency, the current item numbers differ from an older revision chart, and the 2000 wattage prose is malformed. Newer 2025 hose and sprinkler notices also mean older carton art should not be treated as a current contents list. Verify the actual item and nameplate, then measure cabinet, hose, valve and media-removal clearance.

The range alone does not tell you what fish can be stocked, how much food enters the water, or how often mechanical media will clog. A 20-gallon aquarium with high waste production is not the same filtration problem as a lightly loaded tank of the same volume. The manufacturer support material notes that added or growing fish, leftover food and excrement can raise a filter's load and maintenance frequency. It does not provide a capacity score for a particular species or stocking density.

Build a separate livestock plan

Before treating a model as an option, list the animals, their adult size, feeding schedule and waste pattern. Note delicate species, fry, shrimp or other inhabitants that may affect intake protection and outlet flow. If the tank is densely stocked, contains a high-waste animal or has an unusual husbandry requirement, ask a qualified aquarium professional or use species-appropriate care references. None of the 22 individually reviewed Fluval 07, FX, U Series, Nano, current AC and earlier AquaClear models has a verified manufacturer high-bioload rating in the sources checked here, so the finder fails that essential evidence request.

Do not compensate for unknown load by automatically selecting the largest pump figure. Fluval's table separates pump output and filter circulation, and the latter has an equal-length-hose measurement condition. Those are product specifications, not a prediction of real flow through your exact plumbing and media. A flow-control valve is also a bounded adjustment, not a personalized stocking recommendation.

Pick the filter arrangement around the room

Filter size begins with whether the device can be installed at all. The 07 Series is an external canister. Its manual requires the body to sit upright below the aquarium, within the model-specific maximum water-column distance. It also requires a direct hose route without loops and little slack. Check cabinet height, side access, hose path, distance to the waterline, electrical outlet and space to lift the basket assembly. If there is no suitable below-tank position, the 07 family is the wrong form regardless of volume.

The FX family is also gravity-fed: its manual says never to place the unit above water level and recommends that it sit completely beneath the aquarium for best performance. It has a different placement diagram and different hose limits from the 07 models; do not copy the 07 base-to-water maximum into an FX plan. The FX4 and FX6 also need a reachable drain-valve route, while the FX2 drains manually.

The U-series and Nano internal models avoid external canister cabinet placement but occupy aquarium space; U2/U3/U4 have the family manual’s own two-ended intervals, whereas U1 and Nano retain unknown lower bounds. The current AC20/30/50/70/110 hang-on filters also avoid a below-tank cabinet but need a compatible rim, space behind the aquarium, and a waterline near the return. The exact manufacturer limits differ: AC20/30/50 use a 2.75-inch rear gap and 5/8-inch waterline gap; AC70/110 need more wall clearance and permit a larger waterline gap. Read the exact product page and manual before relying on the range.

Use media volume only for the question it answers

The 07 Series basket volumes increase from 1.2 L (107) to 1.8 L (207), 3.1 L (307) and 4.2 L (407). That can help compare media-storage space. It does not establish biological processing capacity for a given load, and it should not be confused with total chamber volume or the full canister volume. Fluval gives separate numbers for each of those spaces. A minimum-media setting in the finder simply excludes a model whose published basket volume is below the chosen minimum; it cannot rank filter biology or maintenance quality.

Finish with the care plan

Look beyond initial setup to cleaning and replacements. The manual schedules recurring tasks for the impeller, intake, hoses, seals and media and says not to replace every medium at once. Check replacement availability and the exact media set included with the seller's model code. A product page may identify the standard box, but retailer bundles can differ; confirm the exact contents, return terms and model suffix at checkout.

The soundest shortlist is the one that fits actual volume, species and load advice, installation, media needs and care tolerance together. A model card is the beginning of that process, not a certification that an aquarium is biologically balanced.

Primary references: Fluval 07 Series, 07 Series manual, 07 Series FAQ, Fluval FX Series, FX Series manual, Current Fluval AC Series, Earlier AquaClear archive and AC Series / earlier-generation FAQ. Aquael comparisons use the ULTRA FILTER US table, the ULTRAMAX US table, the English ULTRAMAX manual, and Aquael's 2023 revision, 2025 hose and sprinkler change notices.