Best Filter for a 5 Gallon Fish Tank
A 5 gallon tank is small, which means water quality swings fast. A good filter keeps your cycle stable and your fish alive. A bad filter either blows them around or does nothing. Here are two solid options.
Our Picks
AquaClear 20 Power Filter
Best OverallThe AquaClear 20 is technically rated for 5-20 gallons, which puts a 5 gallon tank at the very bottom of its range. That sounds like overkill, but the adjustable flow knob is the key. Dial it down to the lowest setting and you get gentle circulation with the best biological filtration on the market for this size. The media basket holds a sponge, carbon, and bio rings instead of disposable cartridges. You keep your bacteria colony when you do maintenance instead of throwing it in the trash.
Pros
- • Adjustable flow rate lets you dial back for bettas and shrimp
- • Customizable media basket with no proprietary cartridges
- • Reliable motor that runs for years
- • Provides mechanical, chemical, and biological filtration in one unit
Cons
- • Even at minimum flow, it may be too strong for long-finned bettas without a baffle
- • Takes up space behind the tank; needs a few inches of clearance from the wall
- • Startup rattle after water changes until it reprimes
Aquaneat Aquarium Bio Sponge Filter
Budget PickA sponge filter is the simplest filter you can run on a small tank. Air goes in, water gets pulled through the sponge, bacteria colonize the sponge surface, toxins get processed. No moving parts in the water. No intake tube that swallows baby shrimp. The flow is gentle enough for a betta to ignore completely. You need a separate air pump to run it, but the total cost is still under $15.
Pros
- • Zero risk to shrimp, fry, or long-finned fish
- • Almost completely silent with a decent air pump
- • Cheap to buy and cheap to run
- • Doubles as supplemental aeration since it runs on air
Cons
- • Requires a separate air pump and airline tubing
- • Less mechanical filtration than a HOB, so water may look less polished
- • Takes up space inside the tank, which matters when you only have 5 gallons
How to Choose a Filter for a 5 Gallon Tank
The flow rate math is simple. Your filter should turn over the tank volume 4-6 times per hour. For a 5 gallon tank, that means 20-30 GPH. The AquaClear 20 maxes out at 100 GPH, which is way too strong at full blast. Turn it down. If you are keeping a betta, you want closer to 20 GPH. Bettas come from slow-moving water and strong current exhausts them.
Three filter types work at this size: sponge, hang-on-back (HOB), and small internal filters. Canister filters exist for nano tanks, but they are expensive and unnecessary for 5 gallons. Save your money.
Sponge filters are the best choice if your priority is livestock safety. Nothing gets sucked in. The flow is gentle. The sponge becomes a biological filtration powerhouse after a few weeks of cycling. Downsides: they sit inside the tank taking up space, and they do not polish the water as well as a HOB.
HOB filters give you the best overall filtration. They pull water through mechanical, chemical, and biological media, then return clean water to the tank. The AquaClear 20 is the gold standard here because the media basket is customizable. Most other HOBs use disposable cartridges that force you to throw away your bacteria colony every month.
If you are running a betta tank with a HOB, consider adding a pre-filter sponge over the intake and a baffle on the output. The pre-filter prevents fin damage and the baffle diffuses the flow so your betta is not fighting a river current.
Regardless of which filter you pick, never replace all the media at once. Rinse the sponge in old tank water. Replace one piece at a time if something is falling apart. The bacteria living in that media are what keep your fish alive.