Match the size to your aquarium volume using the table below. These are Fluval's maximum-rated volumes — for a heavily stocked tank, sizing down to the next size up from "just fits" gives a better turnover rate than running right at a filter's rated ceiling.

U1 ships with a single foam pad plus BIOMAX. U2, U3 and U4 add a second foam pad and two poly/carbon cartridges for a heavier mechanical and chemical load — one reason to size up even on a smaller tank if your stocking is heavy.

• Three-stage filtration — foam mechanical stage, poly/carbon chemical stage (U2-U4), and BIOMAX biological media, in one compact unit.
• Adjustable flow control — top outlet for surface circulation, spray bar for gentle distribution, or bottom outlet for deep-water agitation depending on the model.
• Flip-top lid — lifts for cartridge access without disconnecting the filter from the glass.
• Mounts horizontally or vertically — fits nano cubes and taller tanks alike.
• Runs in freshwater, marine or reptile/paludarium setups — the same unit works across water types.

Turnover (how many times the filter cycles the tank's full volume per hour) falls as you approach a size's rated maximum. At the top of its rated range, each size delivers roughly: U1 around 4.5x/hr at 55 L, U2 around 3.6x/hr at 110 L, U3 around 4x/hr at 150 L, U4 around 4.2x/hr at 240 L. For community or lightly stocked tanks, running at the rated maximum is fine; for a heavier bioload, sizing up gives more headroom.

Suitable for nano and starter freshwater tanks, marine tanks, and reptile or paludarium water features, at or under the rated volume for the size chosen. Not intended as the sole filtration on a heavily stocked or high-bioload tank at the top of its rated range — pair with a second filter or step up a size rather than push one unit past its comfortable turnover.

Filter unit with suction-cup mounts, foam pad(s), BIOMAX biological media, and — on U2, U3 and U4 — two poly/carbon cartridges. U1 ships with one foam pad and BIOMAX only.

Rinse the foam pad in tank water (not tap water) every 2 to 4 weeks to preserve the biological colony. Replace the poly/carbon cartridge, where fitted, once it stops improving water clarity — Fluval does not publish a fixed replacement interval. BIOMAX itself is reusable and doesn't need routine replacement.

Covered by Fluval's manufacturer warranty against faulty parts, in addition to the consumer guarantees you're entitled to under Australian Consumer Law. Standard parcel delivery.

Can I run a Fluval U as my only filter?

Yes, provided your tank is at or under the model's rated volume and stocking is moderate. For heavier stocking, size up or run a second filter alongside it.

Do U1 and U2 use the same media?

No. U1 ships with a single foam pad and BIOMAX only; U2, U3 and U4 add a second foam pad and two poly/carbon cartridges for extra mechanical and chemical filtration.

Will a Fluval U work in a reptile or paludarium water feature?

Yes — Fluval rates the U-series for freshwater, marine and reptile/paludarium use.

Can I mount it horizontally in a low tank?

Yes, the U-series mounts either horizontally or vertically depending on the water depth and layout you need.

What's the difference between the rated flow and what I'll actually see?

The rated flow is measured clean, with the supplied media fitted but before any build-up. Expect it to ease off between cleans, which is normal — regular rinsing of the foam pad keeps it closer to the rated figure.