What Size Filter Does My Aquarium Need?

The filter is the engine room of an aquarium. It is not just moving water around, it is home to the bacteria that turn toxic fish waste into harmless nitrate. Undersize it and the tank never stays clean; oversize it in the wrong way and you blast your fish across the tank. Sizing a filter well means matching its flow to your tank’s volume, then adjusting for the fish you keep.

The good news is that filters are rated by flow, and flow is easy to compare against your tank once you know the target. This guide explains turnover, the number that actually matters, why the rating on the box is optimistic, and how to land on the right filter every time.

The quick answer

Aim for the filter to move your entire tank volume 4 to 6 times per hour for a typical community tank, and up to 10 times for messy or heavily stocked fish like goldfish.

• 100 litre community tank: roughly 400 to 600 litres per hour (LPH)
• Messy or heavily stocked: up to 1,000 LPH

To match a filter to your exact tank, the aquarium filter flow calculator shows the recommended flow range in both litres and gallons per hour. If you are not sure of your tank’s volume, measure it first with the fish tank volume calculator.

Turnover: the number that matters

Turnover is how many times the filter cycles the whole tank volume in one hour. It is the honest way to size a filter, because it scales with your tank rather than with marketing.

Multiply your water volume by the turnover you want. A 120 litre community tank at 5x turnover needs about 600 LPH of real flow. A 120 litre goldfish tank, which produces far more waste, wants closer to 8x to 10x, so 960 to 1,200 LPH. Same tank, very different filter, because the fish set the demand.

Setup	Turnover per hour	Flow for a 100 L tank
Lightly stocked / planted	4x	400 LPH
Standard community	5 to 6x	500 to 600 LPH
Heavily stocked	6 to 8x	600 to 800 LPH
Goldfish / cichlids / messy	8 to 10x	800 to 1,000 LPH

Why the number on the box is optimistic

Manufacturers measure flow with an empty filter and no head height, which is not how yours will run. Once you add filter media, and the filter has to push water up to the tank, real flow drops noticeably, often by 20 to 30 percent for a canister with full media.

The practical fix is to buy a filter rated 10 to 20 percent above your target, so it still hits the flow you need when it is full of media and doing real work. The filter flow calculator gives you the target range, and you simply choose a filter rated a little above the top of it.

More flow is not always better

Strong flow suits rivers species and messy fish, but it exhausts weak swimmers. Bettas, fancy goldfish and other long-finned fish struggle in a strong current and end up hiding or pinned against the glass. This is a common mistake, covered in our guide on what size tank a betta needs, where gentle flow matters more than raw power.

If your filter is too strong, you do not need a new one. Soften the output with a spray bar, a flow deflector, or by aiming the outlet at the glass. You keep the biological filtration and lose the wind tunnel.

Filter types, briefly

• Hang-on-back (HOB): easy, cheap, great for tanks up to about 150 litres.
• Canister: high capacity and quiet, ideal for larger or heavily stocked tanks.
• Sponge filter: gentle and cheap, perfect for betta tanks, fry and shrimp where soft flow is a feature.
• Internal: compact, good for smaller tanks and quarantine setups.

Whatever the type, size it by turnover first, then pick the format that suits your tank and maintenance style.

Do not forget the water volume

Turnover is based on your actual water volume, not the tank’s rated size. Substrate, rocks and decor all reduce the water the tank holds, so a 100 litre tank might really hold 85 litres once it is set up. Work out the true figure with the water volume calculator so your turnover target is based on the water that is actually there.

Frequently asked questions

What GPH filter do I need for my tank? Aim for 4 to 6 times your tank volume per hour for a community tank, and up to 10 times for messy fish. For a 20 gallon (75 litre) tank that is roughly 80 to 120 GPH, up to 200 GPH for goldfish.

Is a higher GPH filter better? Only up to a point. Higher flow helps messy tanks but pushes weak swimmers around. Match the flow to your fish, and soften it with a spray bar if needed.

Why is my filter’s real flow lower than the rating? Ratings are measured with an empty filter and no lift. Media and head height reduce real flow, often by 20 to 30 percent, so buy a filter rated slightly above your target.

What size filter for a goldfish tank? Goldfish are messy, so aim for the high end, around 6 to 10 times turnover per hour, with plenty of biological media. Use the filter flow calculator for the exact range.

Can I run two filters on one tank? Yes, and it is a good idea on larger or heavily stocked tanks. Two filters add capacity and keep the tank stable if one needs cleaning or fails.

The bottom line

Sizing a filter is really one calculation: your water volume times the turnover your fish need. Get the volume from the fish tank volume calculator, pick a turnover from the table above, and let the aquarium filter flow calculator turn it into a flow range in litres and gallons per hour. Buy a little above the target to allow for media, and soften the output if your fish prefer calm water.