Yes, a fish tank needs a filter. It traps debris, grows the bacteria that remove ammonia, and can polish the water, and the bacteria job is what keeps fish alive. Pick a type by tank size, rate it for your litres, and rinse the media in old tank water, never under the tap.
Does a fish tank need a filter? Yes, almost every time. A filter traps waste and houses the bacteria that turn toxic ammonia into nitrate, and RSPCA Australia says lacking an effective biological filter is the number one cause of fish deaths in aquariums. The few filterless tanks that work are heavily planted, lightly stocked and run by experienced keepers.
A filter also moves water, which brings oxygen to fish and to the bacteria living in the media. Setting up a tank? Filters are one of the 12 items on the what you need for a fish tank checklist, and the Filters hub has more guides.
1. Does a Fish Tank Need a Filter? The Short Answer
For a beginner, yes, always. Fish produce ammonia constantly, and in a closed tank it has nowhere to go without bacteria to convert it into nitrite and then nitrate. Those bacteria need a surface to live on and a steady flow of oxygenated water, which is exactly what filter media provides. Read how to cycle a fish tank to see how that colony gets established.
Does a Fish Tank Need a Filter If It’s Heavily Planted?
There are exceptions, but they sit at the extreme ends of the hobby. A heavily planted tank with very few fish can have plants use up the ammonia, and some experienced keepers run tanks that way. It takes skill, and a missed water change or a dying plant can tip the balance fast. If you’re asking the question, use a filter.
What a Filter Actually Does
Filters do up to three jobs. Only one of them is non-negotiable.
| Job | How it works | How much it matters |
|---|---|---|
| Biological | Bacteria on the media convert ammonia to nitrite, then nitrite to nitrate | Essential. RSPCA Australia calls it the most important of the three |
| Mechanical | Sponge or floss traps particles so you can remove them | Important for clear water and for keeping the media from clogging |
| Chemical | Media such as carbon absorbs certain dissolved substances | Optional, useful for a specific problem |
A tap-water rinse can undo the biological job in minutes. RSPCA Australia says to rinse filter media in a bucket of tank water when it clogs or about monthly, whichever comes first, because chlorine kills the beneficial bacteria. Over-cleaning removes them too.
Filter Types Explained
| Type | How it works | Suits | Watch out for |
|---|---|---|---|
| Sponge | Air-driven, so water is drawn through a sponge that traps debris and hosts bacteria | Small tanks and fish that like gentle flow | Needs an air pump; little room for chemical media |
| Hang-on-back | Hangs on the rim; water is lifted through a media box and spills back in | Small to medium tanks; easy to service | Needs rim space and a lid cut-out |
| Internal | Sits submerged, with a small motor pulling water through built-in sponge | Small to medium tanks, tight budgets | Takes up space in the tank; limited media |
| Canister | Sits below the tank; hoses carry water through a sealed media canister | Medium to large tanks needing plenty of media | More setup, and servicing is messier |
Sumps, which run water through a separate tank below, are a larger and more advanced setup, mostly seen on big or marine tanks. See the roundups for sponge filters, internal filters, canister filters and small or nano tank filters, plus the general filter roundup.
Sizing and Looking After Your Filter
- Rate it for your litres. Check the tank size on the box, and work out how many litres you really have first. Real flow is usually lower than the label, so many keepers choose a filter rated at or above their tank size.
- Match the flow to the fish. Fish that prefer low flow, such as Siamese fighting fish, need a filter with adjustable flow, according to RSPCA Australia. See the Betta Fish guides. Keeping some species is restricted in parts of Australia, so check your state’s rules before you buy fish.
- Keep it running. The bacteria need oxygen, so RSPCA Australia treats aeration as critical for a healthy biofilter.
- Clean the impeller too. RSPCA UK says to swish the media in removed tank water and clean the impeller regularly, never under the tap.
Signs Your Filter Isn’t Coping
Watch for flow that has slowed, a rattling motor, cloudy water or an ammonia reading above 0. Clean the media the RSPCA way first, then read how to reduce ammonia in a fish tank if the reading stays up.
Frequently Asked Questions
Can fish live without a filter?
Not reliably. A few heavily planted, lightly stocked tanks manage it, but ammonia builds up quickly without one. Beginners should always use a filter.
What does a filter do in a fish tank?
It traps waste, moves water, and grows the bacteria that turn ammonia into nitrite and then nitrate. That biological job matters most, according to RSPCA Australia.
Which filter type is best for a beginner?
It depends on tank size. Sponge or internal filters suit small tanks, hang-on-back suits small to medium ones, and canisters suit bigger tanks. Pick one rated for your litres.
What size filter does my fish tank need?
One rated for at least your tank’s volume in litres, since real flow is often below the label. Confirm your real volume first, then choose a filter at or above that rating.
How often should I clean my filter?
RSPCA Australia says to rinse the media in a bucket of tank water when it clogs or monthly, whichever comes first. Never use tap water, and don’t clean everything at once.
