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Glass & Gravel
Roundup

The best nano tank filters

Ranked on whether the flow can be brought under four turnovers an hour, and on whether you get to keep the bacteria you grew.

By Scooter M., Enthusiast. Published September 1, 2026.

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Water circulating at the surface of a small planted aquarium
Quick picks

The shortlist, ranked

Ranked on published manufacturer specifications and, where the figures exist, on the arithmetic they imply for a tank this size. Tap any row for the full write-up, including who should skip it.

#ProductBest forScorePrice
01Aquarium filtration equipment of the type described - a stand-in for the Fluval Nano Aquarium Filter, not a photograph of this modelFluval Nano Aquarium FilterA betta tank where the flow has to be gentle from day oneBest in-tank8.6Check price on Amazon
02Aquarium filtration equipment of the type described - a stand-in for the Nano sponge filter with air pump, not a photograph of this modelNano sponge filter with air pumpA betta tank, a shrimp tank, or a quarantine tank on a budgetBest on a budget8.1Check price on Amazon
03Aquarium filtration equipment of the type described - a stand-in for the Fluval AquaClear 20 Power Filter, not a photograph of this modelFluval AquaClear 20 Power FilterA 10-gallon where you want media capacity, not a cartridgeBest hang-on-back8.3Check price on Amazon

Scores are ours and editorial. They are never published as review or rating markup, because we have not tested these products.

The finding, before the picks

Not one filter in this roundup publishes a flow rate. Not the Fluval Nano, rated only “up to 15 US Gal / 60 L”. Not the AquaClear 20, rated “5-20 US Gal / 18-76 L”. Sponge filters do not even have a manufacturer to ask.

So a buyer is expected to match a filter to a tank using a range spanning a factor of four, with no figure to compute turnover from. We record that absence in the specification table of every product below rather than filling it with a number we would have had to invent, and it is the single largest reason this category is hard to shop for honestly.

What we ranked on instead

Whether the flow can be brought under our ceiling. An adjustable output, a spray bar, an air valve, or somewhere to fit a baffle. 4.3 gal of actual water x 4 turnovers an hour is about 17.2 GPH. Past roughly 25.8 GPH you are baffling the outflow or buying a different filter.

Biological capacity, and whether you keep it. Rinsable foam and ceramic media score. Disposable cartridges do not - replacing one throws away most of the colony you grew.

Running cost over three years. Not the shelf price. A cheap cartridge filter costs more than the tank over its life.

What the maker publishes. A rated range with no flow figure is half a specification and is marked as such.

Sizing, if you can find a GPH figure

Tank (rated)Actual waterTarget flow (4x)Ceiling (6x)
3 US gal2.6 gal10.4 GPH15.6 GPH
5 US gal4.3 gal17.2 GPH25.8 GPH
10 US gal8.5 gal34 GPH51 GPH
20 US gal17 gal68 GPH102 GPH

Why the sponge filter is second and not first

On the merits it is arguably the best filter here: no impeller, infinitely adjustable, enormous biological surface, and it survives a power cut with its bacteria intact because it never sits dry. If you are keeping shrimp it is the only sensible choice.

It is second because it has no manufacturer. These are sold under a dozen interchangeable brand names with no published specifications of any kind, no accountability and no consistency between batches. On a site whose entire method is reading what manufacturers publish, that has to cost something - and we would rather explain the ranking than quietly hide the problem.

What is not here

Cartridge filters. The category that ships with a proprietary floss-and-carbon cartridge you replace monthly. They are the most-sold nano filters on the market and we will not recommend one: every replacement throws away the bacterial colony, so the tank never fully establishes, and over three years the cartridges cost more than the filter.

Internal power filters with no flow control. A pump you cannot turn down in a tank the fish cannot leave is a permanent problem.

The write-ups

Every pick, in full

01 / Best in-tankAquarium filtration equipment of the type described - a stand-in for the Fluval Nano Aquarium Filter, not a photograph of this model

Fluval

8.6 / 10

Fluval Nano Aquarium Filter

An in-tank foam filter rated to 15 gallons, with an adjustable output and a spray bar in the box. The spray bar is why it is here: it is the only nano filter in this list that ships with the part that turns a jet into a sheet, which is exactly what a long-finned betta needs. Fluval does not publish a flow rate for it, and we are not going to invent one.

Check price on Amazon

#ad We hold no verified live price for this product, so we show none. How this site is funded.

Published specifications for the Fluval Nano Aquarium Filter, with the page each figure was read from
SpecificationValueSource
Rated aquarium capacityup to 15 US Gal / 60 LFluval published specifications
Flow rateNot published on the product page - Fluval rates this filter by tank size only, so you cannot compute turnover from the boxAbsence recorded by Glass & Gravel
Media1 Bio-Foam pad and 1 fine foam pad included; carbon cartridge sold separatelyFluval published specifications
Flow controlAdjustable flow output with attachable spray bar, described by Fluval as safe for delicate fish or shrimpFluval published specifications
Mounting3 suction cups, in-tankFluval published specifications

What it does well

  • Spray bar included, not sold separately
  • Foam media means real biological surface, not a disposable cartridge
  • Adjustable output, and Fluval says outright it suits delicate fish
  • Cheap to keep running - foam rinses, it does not get replaced monthly

What it does not

  • No published flow rate anywhere on the product page
  • In-tank, so it occupies display volume in a tank that has none to spare
  • Suction cups on glass are a maintenance annoyance forever

Skip this one if

You want the equipment out of the display. This sits in the tank on three suction cups and there is no hiding it.

How it scores, by criterion

Published spec depth5.5
Flow suitability for a betta9.5
Biological capacity8.5
Running cost9.0
Concealability4.5
02 / Best on a budgetAquarium filtration equipment of the type described - a stand-in for the Nano sponge filter with air pump, not a photograph of this model

Generic (multiple sellers)

8.1 / 10

Nano sponge filter with air pump

The cheapest correct answer for a betta, and the one we would fit if we could only fit one thing. A sponge filter has no impeller, no outflow jet and no way to shred a fin. It is also the only filtration in this list that keeps working when the power comes back on after an outage without needing to re-prime. What it does not have is a manufacturer: these are sold under a dozen interchangeable brands with no published specifications at all, and we are not going to pretend otherwise.

Check price on Amazon

#ad We hold no verified live price for this product, so we show none. How this site is funded.

Published specifications for the Nano sponge filter with air pump, with the page each figure was read from
SpecificationValueSource
Flow rateNot published on the product page by any seller in this categoryAbsence recorded by Glass & Gravel
Rated aquarium capacityVaries by seller and is not traceable to a manufacturer document in this categoryAbsence recorded by Glass & Gravel
MechanismAir-lift through open-cell foam; no impellerCategory description
RequiresAn air pump and airline, usually sold separatelyCategory description

What it does well

  • No impeller, so it physically cannot injure a fin or a shrimp
  • Flow is set by the air pump, so it is trivially adjustable with a valve
  • Enormous biological surface for the money
  • Survives a power cut without losing its bacteria to a dry chamber

What it does not

  • No manufacturer, no published specifications, no accountability
  • Air pump noise is a real consideration on a desk
  • Visible black sponge in a small display
  • Zero mechanical polish - the water stays slightly less crystalline

Skip this one if

You cannot live with an air pump humming on your desk, or with a black sponge in the corner of the display.

How it scores, by criterion

Published spec depth1.0
Flow suitability for a betta10.0
Biological capacity9.0
Running cost9.5
Concealability3.5
03 / Best hang-on-backAquarium filtration equipment of the type described - a stand-in for the Fluval AquaClear 20 Power Filter, not a photograph of this model

Fluval

8.3 / 10

Fluval AquaClear 20 Power Filter

The hang-on-back that everyone eventually buys, rated 5-20 gallons. Its advantage is the media basket: you choose what goes in it, and you can pack it with foam and BIOMAX and skip the carbon entirely. Its problem in a nano is that a 5-gallon sits at the very bottom of its range, so you will be running the flow control near minimum and probably baffling the waterfall as well.

Check price on Amazon

#ad We hold no verified live price for this product, so we show none. How this site is funded.

Published specifications for the Fluval AquaClear 20 Power Filter, with the page each figure was read from
SpecificationValueSource
Rated aquarium range5-20 US Gal / 18-76 LFluval published model range
Flow rateNot published on the product page on the AquaClear 20 product pageAbsence recorded by Glass & Gravel
Media includedAquaClear Foam, Activated Carbon, BIOMAXFluval published specifications
Flow controlPatented adjustable flow controlFluval published specifications
Item numberA595FFluval published specifications

What it does well

  • Open media basket - you decide what filtration you run
  • Foam and BIOMAX included, so the biology is there on day one
  • Flow control is genuinely usable, not a token slider
  • Media is cheap and universally stocked

What it does not

  • No published GPH figure
  • A 5-gallon is the very bottom of its rated range
  • The waterfall return needs baffling for a long-finned betta

Skip this one if

The tank is 3 gallons. The bottom of AquaClear's published range is 5 gallons and there is no honest way to read that as covering a 3.

How it scores, by criterion

Published spec depth5.0
Flow suitability for a betta6.5
Biological capacity9.5
Running cost8.5
Concealability7.5

How these picks were chosen

Everyone in this category says they tested twenty tanks. We have not tested any, and we say so. What we did instead: read the manuals, compile the published specifications, compute the wattage and the turnover against the tank’s real water volume rather than its shelf label, and show the arithmetic so you can check it.

The full method, including what we refuse to claim.

Questions

Frequently asked

What size filter for a 5-gallon betta tank?

About 17.2 GPH, with a ceiling around 25.8. In practice you will not find a nano filter that publishes its GPH, so buy one rated at or slightly above five gallons with a flow control, and turn it down.

Are sponge filters good enough on their own?

For a betta or shrimp tank under ten gallons, yes. They provide excellent biological filtration and adequate mechanical filtration. What they do not do is polish the water to crystal clarity, and they do not remove dissolved organics the way carbon does - neither of which a betta cares about.

How often should I clean a nano filter?

When the flow visibly drops, typically every two to three months. Rinse the media in old tank water in a bucket, never under the tap. Never replace all the media at once.

Do I need carbon in my filter?

No, not routinely. Carbon removes dissolved organics and medications and is exhausted within weeks, after which it is just an expensive place for bacteria to live. It is genuinely useful after dosing a medication and for removing tannins from new driftwood. The rest of the time, more foam is a better use of the space.

Sources

Where the figures on this page came from

  1. Fluval Nano Aquarium Filter - published specifications
  2. Fluval AquaClear 20 Power Filter - published model range
  3. Seachem Stability - published dosing directions
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