The best filter for a planted tank

The specification that wins this a planted-tank page is the published minimum: 62 GPH against a 119 GPH maximum. Almost no maker in this class tells you where the bottom of the flow range lands, and in a planted tank the bottom of the range is where you will be running.

The five stages are a genuine difference rather than marketing. Two mechanical, one chemical and two biological, the last of which is a trickle chamber where water passes over media in air rather than submerged — the oxygen-rich environment nitrifying bacteria actually want. Fluval also describes a refiltration system that slows the output and gives a gentle flow, which is the opposite of what most hang-on-back filters do to a water surface.

In practice that combination gives you a filter that polishes the water, holds real biological volume, and returns it without a waterfall. In a planted tank that last point is the one that matters.

The honest costs. The stated 30 gallon range is optimistic: 119 rated GPH is well under the 257 our method asks for on a fully stocked 30, and under the 170 a stocked 20 gallon community tank wants, so read that figure as light stocking. The poly/foam pad is a consumable rather than a rinse-and-reuse foam. No pre-filter sponge is included, which a shrimp tank wants. It is physically taller than a basic hang-on-back and harder to hide behind a background. And five stages means five things to understand before your first maintenance.

The argument for this over the C2 is capacity. A planted tank still has to turn its water over, and on a 20 to 30 gallon tank 150 GPH does that where 119 is stretching.

The basket is the second argument. A cartridge filter decides your media for you — mechanical and carbon, replaced on a schedule. A basket lets you decide, and in a planted tank what you want in that slot is biological volume, not carbon.

Third, the pre-filter sponge on the intake is included rather than sold as an accessory. That is the part that stops a trimmed leaf, a snail or a shrimp disappearing into the impeller, and it is the one thing the C2 does not supply.

The honest costs. The cascade return is a surface-churner by design, and quieting it means either turning the flow down or keeping the water level high so the return does not fall through air — more fiddling than the C2 asks for. At full output on a 10 gallon tank it is more current than a planted nano wants, so the knob is not optional there. No published minimum flow figure, so you cannot check where the knob lands. The rated figure is measured with clean media and no head height, so the real-world number is lower; our method divides by 0.7 to allow for it. And it is a box hanging on the back of a tank you have presumably spent some effort making look good.

In a planted nano, the filter's job is mostly biological, because the plants are doing the nutrient work and there is very little mechanical debris to clear. A sponge filter does exactly that job and nothing else.

It also solves the two problems a planted tank has with powered filters. There is no impeller to mince a shrimp or a loose leaf, and the uplift column lifts water gently rather than firing it across the surface.

A spare sponge living in an established tank is also the cheapest insurance in the hobby: squeeze it into a quarantine tank and you have instant cycled filtration.

The honest costs. No pump, airline or check valve in the box, so the real purchase is four items, and the pump is the noisiest thing in the setup. It gives you no mechanical polish, so fine suspended particles stay suspended. It sits inside the tank in full view, which is an aesthetic cost in a tank you are planting for looks. And it is a nano device: a stocked, planted 20 gallon community tank wants more than this on its own.

The canister argument on a planted tank is not flow, it is placement. Everything except an intake pipe and a return pipe lives in the cabinet, and you choose where those two pipes point. Aim the return along the glass instead of at the surface and you get circulation without churn, which is the single hardest thing to arrange with a hang-on-back.

The media volume is the second argument. Stacked baskets give you room for far more biological media than any hang-on-back basket, and in a planted tank where you are feeding bacteria and plants rather than running carbon, that volume is the useful kind of capacity.

The honest costs. It is several times the price of a hang-on-back and the most expensive pick here by a wide margin. Maintenance is a disconnect-and-carry job rather than a lift-the-lid job, and a canister you dread servicing is a canister that goes six months between cleans. There are more failure points — seals, O-rings, hose clips — and every one of them is a potential leak under your tank rather than in it. And Fluval's own 30 gallon figure is, again, a light-stocking ceiling rather than a working one.

This is the pick that changes the most for the least money, and it is not a filter at all.

Most hang-on-back filters ship with a mechanical pad and a bag of activated carbon. Carbon has real uses — pulling tannins, clearing medication after treatment — but it is not something a settled planted tank needs running permanently, and the slot it occupies is the best biological real estate in the filter.

Swap it for a porous biological medium and the filter holds a larger bacteria colony in the same housing. In a planted tank, where plants are consuming nitrogen and you want stability rather than chemical intervention, that is the right use of the space.

The honest costs. It is not a mechanical medium, so you still need a pad or sponge in front of it or it clogs with debris that should have been caught earlier. It does nothing visible — there is no way to see that your biological capacity went up, which makes it an unsatisfying purchase. Rinse it in tank water only; tap water kills what you are paying for. And carbon is genuinely the right answer after medicating, so do not throw it away.

The skip entry is a whole category rather than a bad product: cartridge-based filtration, of which this is a perfectly competent example.

A cartridge is mechanical media with carbon sewn inside it, and the instructions tell you to replace it monthly. Every replacement throws away the bacteria living on it. On a fish-only tank with a bio-wheel or a second media stage that is survivable. On a planted tank it is working against the stability that plants and bacteria are both trying to give you.

It is also a recurring cost that never ends, which is the quiet argument for a basket filter at any tank size.

If you already own a cartridge filter, the sane fix is not to buy another filter: rinse the cartridge in tank water rather than replacing it until it is physically falling apart, and stuff biological media behind it if the housing has room.

The honest costs of the skip. Cartridges are convenient, which is why they sell, and a reader who will genuinely not do media maintenance may get better results from a cartridge they replace than from a basket they ignore. The six-pack is also cheaper per month than it looks. The objection is specific to planted tanks, not universal.