AquaClear 20 review
The hang-on-back most nano keepers end up owning, and the one whose single best feature is an empty plastic box.
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The short version
The AquaClear 20 is a hang-on-back filter rated by Fluval for 5 to 20 gallon aquariums, and its defining feature is not the pump - it is that the media chamber is an empty basket you fill yourself. Almost every competitor at this price sells you a sealed cartridge and a monthly re-buy. This sells you a box.
That one design decision is why it is on this site. It is also why the review below spends more time on what goes inside it than on the filter itself.
What Fluval publishes
| Specification | Published value |
|---|---|
| Rated aquarium range | 5-20 US Gal / 18-76 L |
| Media included | AquaClear Foam, Activated Carbon, BIOMAX |
| Flow control | Patented adjustable flow control |
| Item number | A595F |
| Flow rate | Not published on the product page |
That last row is the review. Fluval rates this filter by tank size and declines to state a gallons-per-hour figure, which means you cannot compute turnover before buying. Our target for a betta is 4 turnovers an hour with a 6 ceiling, and there is no published number to check that against. We record the absence rather than invent one.
Which tanks it actually fits
Fluval’s range is 5 to 20 gallons. Ranges have ends, and both ends of this one behave badly.
| Tank | Actual water | Turnover target | Verdict |
|---|---|---|---|
| Rated 5 gal | 4.3 gal | about 17.2 GPH | Bottom of the range - flow control near minimum, expect to baffle |
| Rated 10 gal | 8.5 gal | about 34 GPH | The sweet spot - mid-range, dial usable both ways |
| Rated 20 gal | 17 gal | about 68 GPH | Its stated ceiling - buy the AquaClear 30 instead |
A 10-gallon is where we would buy this filter without hesitating. Below that it is more filter than the tank wants; at twenty it is at its published limit with no headroom for the fish you will inevitably add.
The basket is the product
Fluval ships it with foam, activated carbon and BIOMAX. Our standing recommendation is to take the carbon out and fill the space with more foam or more ceramic, because carbon is a tool for a specific job - clearing medication or an odor - and is exhausted within weeks. After that it is an expensive lump occupying basket volume that could be doing biology.
Coarse foam at the intake side, ceramic in the middle, fine foam or floss last. Rinse the foam in old tank water when the flow drops, never under the faucet, and never replace all the media at once. The full media walkthrough is here - it is the difference between a filter that costs nothing to run and one that bills you monthly.
The waterfall, and the ten-minute fix
The return is a waterfall over a weir, and it hits the surface hard enough to push a long-finned betta around in a small tank. The flow control helps and frequently is not enough on its own. The fix is a piece of coarse filter foam wedged in the outflow or a section of plastic bottle clipped over it, which spreads the fall into a sheet. How to baffle it properly covers both. Never restrict the intake instead - that starves the media of the oxygenated flow the bacteria need.
What it is like to live with
Two things are worth knowing before you buy, and both are structural rather than experiential, so we can state them honestly without having run one for a year.
It needs priming after a power cut. A hang-on-back holds water in its chamber; if the level drops while the power is off, the impeller can come back on dry and run without moving water. That is the single practical advantage a sponge filter has over it, and it is why we suggest running one alongside in anything from ten gallons up.
It hangs on the back, so it needs clearance. Measure the gap between the tank and the wall before you order. A tank pushed flush against a wall has nowhere for this filter to go, and that is the commonest reason people end up with an in-tank filter they did not want.
AquaClear 20 or AquaClear 30?
Size by where your tank falls in the published range, not by price. The 30 is rated 10 to 30 gallons; a 20-gallon sits in its middle and a 10-gallon sits at its bottom. So: a 5 or 10-gallon takes the 20, a 20-gallon takes the 30, and a 15-gallon can take either with the 30 giving more media room. Our 20-gallon page works that through in full.
Who should not buy it
Shrimp keepers running a colony, unless the intake carries a sponge pre-filter - an impeller does not distinguish a shrimplet from detritus. Anyone whose tank has no clearance at the back. And anyone with a 3-gallon: Fluval’s range starts at 5 and there is no honest way to read that as covering a 3.
What we cannot tell you
The real flow rate, because Fluval does not publish one. How quiet it is in a bedroom. Whether the impeller is still smooth at year three. Those are ownership questions and we have not owned one - we would rather say so than write a confident paragraph about a noise level we have never heard.
What we would buy for this
Chosen from the same registry as our roundups, on published specifications and the arithmetic they imply - not on testing we have not done.
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.
Skip it 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.
Fluval AquaClear 30 Power Filter
The next size up in the same range, sold for 10 to 30 gallon tanks, and the one that puts a 20-gallon in the middle of its published range instead of at the top. Same open media basket as the 20, same genuinely usable flow control, same missing GPH figure. If you are choosing between the 20 and the 30 for a 20-gallon, the honest answer is the 30 - the 20 tops out exactly where your tank starts.
Skip it if: The tank is 5 or 10 gallons. The bottom of this model's published range is 10 gallons and its minimum flow is more than a nano needs.
Frequently asked
What size tank is the AquaClear 20 for?
Fluval publishes 5 to 20 US gallons. In practice a 10-gallon is the sweet spot: a 5-gallon puts you at the bottom of the range with the flow control near minimum, and a 20-gallon is its stated ceiling with no headroom.
What is the AquaClear 20 GPH?
Fluval does not publish one. The AquaClear range is rated by tank size only, and we are not going to repeat a figure we cannot trace to a manufacturer document. Our turnover target for a betta in a 10-gallon is about 34 GPH, and you reach it with the flow control and your eyes rather than with a spec sheet.
Should I take the carbon out?
Usually yes. Carbon is exhausted within weeks and then it is just an expensive sponge taking up basket space that would do more good as foam or ceramic. Keep a bag for after medication, and run foam and BIOMAX the rest of the time.
Is the AquaClear 20 safe for a betta?
With the flow turned down and, in a small tank, a baffle on the outflow, yes. The risk is not the intake, it is the waterfall return pushing a long-finned fish around the tank all day.
Is it safe for shrimp?
Only with a sponge pre-filter over the intake. There is an impeller in this filter and it cannot tell a baby shrimp from detritus - our shrimp tank page covers the filtration a colony actually wants.