Photo: Ofkun · CC BY-SA 4.0
filtration
Hang-on-back filter for 10- and 20-gallon tanks
Adjustable HOB filtration for 10s and 20s: reusable foam and ceramic beat disposable cartridges, baffle the outflow for long-fin fish, and expect to reprime after some power cuts.
Size the filter to the waste, not the box art
A hang-on-back filter pulls water up an intake, through a media basket, and returns it as a waterfall. For a 10-gallon, aim for roughly 4–6× turnover with a flow dial you can actually use. For a 20-gallon community, buy a unit rated for 20–30 gallons, the loud kit cartridge that clogs in a week is not filtration, it is a subscription.
Livebearer tanks (swordtails, mollies, platies) and other midwater groups make real waste. Media volume and weekly water changes beat a tiny motor screaming at max. Bettas and long-fin fish want the outflow baffled or dialed down so they are not pinned to the glass.
Foam and ceramic first, carbon optional
Prefer reusable sponge or coarse foam plus ceramic or other bio media over disposable carbon cartridges. Foam catches debris and grows bacteria. Ceramic holds more bio surface. Carbon removes some organics and meds, then saturates, it is not the biofilter, and tossing a combo cartridge every week throws out bacteria you paid to grow.
Squeeze foam in a bucket of old tank water when flow slows. Never tap water. Stagger cleaning so you do not wipe the whole basket in one day. A sponge filter on the same tank can carry bio through a HOB clean, different tool, same nitrogen cycle.
Noise, lids, and power cuts
Rattling lids, vibrating stands, and salt creep in the rim gap cause most 'loud HOB' complaints. Level the filter, seat the impeller, and keep the lid from buzzing. A drip loop in the cord keeps water out of the outlet.
After a power cut, many HOBs with an external motor will not self-prime. Fill the chamber with tank water and restart. That is normal, not a mystery defect. Unplug before a deep water change if the intake will suck air for a long stretch.
The picks below are an adjustable unit for ~10 gallons, a larger basket for ~20, and reusable foam so you can stop buying cartridges. A broader search sits under that if you need another brand. We may earn a commission if you buy. See the affiliate disclosure.
What we would buy
AquaClear 20 Power Filter
Adjustable hang-on-back sized for roughly 5–20 gallons with a real media basket (foam, carbon, BioMax). Flow control lets you soften the waterfall for bettas and long-fin livebearers without throwing the bio media away every week.
Prime the box with tank water on first start. Carbon is optional once the tank is established. Foam and ceramic do the daily work. Skip this if a seeded sponge already holds a light 5–10 and ammonia stays at zero.
See this listingAquaClear 30 Power Filter
The usual step-up for a 10–20 gallon community or a messy livebearer 20. Rated for 10–30 gallons with adjustable flow and enough basket volume that you can keep foam and ceramic instead of disposable cartridges.
On a lightly stocked 10 you can dial flow down; on a swordtail or molly 20, the extra media room is the point. After a power cut, refill the chamber if it does not restart on its own.
See this listingAquaClear 20 foam inserts, 3-pack
Reusable coarse foam is the 'stop buying cartridges' pick. Squeeze it in old tank water during water changes. Match foam size to your HOB model; this pack fits the AquaClear 20 and cuts down for some other baskets.
Never rinse foam under the tap. Buy the foam sized for your filter (30/50 packs exist for larger AquaClears). Extra foam also works as an intake prefilter for fry and shrimp.
See this listing
Frequently asked questions
What size hang-on-back filter for a 10-gallon tank?+
Look for adjustable flow and roughly 4–6× turnover (about 40–60 GPH on a 10). An AquaClear 20-class unit is a common fit. Dial the outflow down for bettas and long-fin livebearers.
What size HOB for a 20-gallon tank?+
Buy a filter rated for 20–30 gallons, not the undersized kit cartridge that clogs in a week. An AquaClear 30-class unit is a common pick for a community 20. Livebearer bioload likes media volume more than a tiny motor on full blast.
HOB vs sponge filter: which do I need?+
Different jobs. A sponge is gentle bio and fry-safe intake for nanos and hospital tanks. A HOB moves more water through a media basket on a 10–20 community. Many 20s run HOB plus a seeded sponge as backup. See our sponge filter page for the air-driven setup.
Why did my HOB stop after a power cut?+
Many hang-on motors sit outside the tank and do not self-prime when the box drains. Fill the filter chamber with tank water, check the intake is seated, and plug back in. A drip loop and a check that the impeller spins free still matter.