Choosing a Filter for a Planted Aquarium

What a filter does in a planted tank

Tropica describes the job of the pump and filter as three things: circulating water, catching fine particles, and housing the bacteria that keep the tank biologically stable. In a planted tank, the first job carries extra weight. Plants take CO2 and nutrients from the water immediately around their leaves. Where water stagnates, that thin layer gets depleted, growth slows, and debris settles. Dead spots behind hardscape or under dense stems are common places for algae to start.

If you are setting up a new tank, the filter is also where most of your nitrifying bacteria will live. Our guide to cycling and water testing explains how that colony gets established.

Which filter type suits a planted tank?

Canister filters sit outside the tank, usually in the cabinet, and connect through intake and outflow hoses. They hold a lot of media, run quietly, and keep equipment out of view, which is why they are common in aquascapes. They take more effort to prime and clean, and a hose or connector leak happens outside the tank, so check the fittings carefully.

Hang-on-back (HOB) filters hang on the rear rim. They are simple to install and service, and the waterfall return adds surface movement. They need a rim or enough glass edge to hang on, and the box is visible behind the tank.

Sponge filters are driven by an air pump. They are gentle, inexpensive, and safe for shrimp and fry, and the sponge itself becomes good biological media. Their flow is modest and the air line and sponge are visible, so they suit small tanks, shrimp tanks, and quarantine setups more than large display scapes.

Internal filters sit inside the tank and pump water through a media cartridge. They are compact and easy to start, but they take up space in the layout and usually hold less media.

How much surface agitation does a planted tank need?

Surface movement controls gas exchange. A rippling surface lets oxygen in and lets CO2 out. That creates a real trade-off.

• In a CO2-injected tank, heavy surface agitation drives off the gas you are paying to inject, so you need more CO2 for the same effect.
• Oxygen still matters. Fish need it, and at night plants also consume oxygen. Very still water in a heavily planted, heavily stocked tank can run low on oxygen overnight.

The common middle ground is a gentle, visible ripple across the surface rather than splashing or a perfectly flat surface. Many keepers in injected tanks keep the outflow slightly below the surface during the day and accept a little more ripple at night, or run an air stone on a timer after the lights and CO2 go off. In a low-tech tank without injection, more surface movement is generally fine and can help, because there is no injected CO2 to lose. Our CO2 system guide explains how to recognize livestock stress from too much CO2.

What size filter does a planted tank need?

Start by choosing a filter rated for at least your tank's real volume. Then treat the flow figure on the box with some skepticism. Manufacturer flow ratings are generally measured with the filter empty and no height for the pump to lift water. Once you add media, hoses, bends, inline equipment, and the height from cabinet to waterline, real flow can be noticeably lower. Media clogging between cleanings reduces it further.

Hobbyists use turnover rules of thumb, expressed as how many times the tank's volume passes through the filter each hour. Tropica recommends circulating the tank volume three to five times per hour, and gives the example of a 150-litre aquarium needing 450 to 750 litres per hour. For a 40-gallon tank (about 150 litres), that works out to roughly 120 to 200 US gallons per hour of real flow, not box flow. Planted-tank keepers running CO2 often aim higher. Treat any such number as a starting point, not a target that guarantees results. The better test is to watch the tank: plants in every area should sway slightly, and debris should not collect in corners.

If you have dense stems, tall hardscape, or a long tank, consider a filter with some headroom over your volume, or a second outflow, rather than a single powerful jet that flattens plants near the outlet.

Lily pipes and inline equipment

Many canister setups use clear glass or acrylic inflow and outflow pipes, often called lily pipes. They look tidy and some outflow designs spread water more widely across the tank. Glass pipes need careful cleaning and handling.

Canister hoses can also carry inline equipment such as an inline CO2 diffuser, an inline heater, or a CO2 reactor. Each adds resistance, so flow drops further. Check that connectors match your hose diameter before buying anything inline.

Media basics

Most filters combine three kinds of media:

• Mechanical media, such as sponges and floss, traps particles. Fine floss clogs quickly and is often replaced rather than rinsed.
• Biological media, such as ceramic rings or porous stones, gives nitrifying bacteria a large surface to live on.
• Chemical media, such as activated carbon, adsorbs some dissolved substances. Many planted-tank keepers leave it out in routine use, because it may also remove some liquid fertilizer components, and add it only for specific jobs such as removing medication.

Water should pass through mechanical media first, so biological media stays clean longer.

How often should you clean a planted tank filter?

Clean the filter when flow drops noticeably rather than on a rigid schedule. Tropica puts it at about once a month, or a little less often, for sponges.

• Rinse biological media in removed tank water, not untreated tap water. One canister maker's manual, for example, advises rinsing biological media in aquarium water and never replacing all of it at once, since chlorine and a full media swap can remove the bacteria that keep the tank cycled.
• Rinse gently. The aim is to restore flow, not to sterilize.
• Never use soap or detergents on media or filter parts.
• Clean the intake, hoses, and pipes occasionally, since biofilm inside hoses reduces flow.
• Unplug the filter before opening it, and check seals and O-rings when you reassemble a canister.

Noise and placement

Plan where the filter will sit before you fill the tank. A canister needs space in the cabinet for the unit and its hoses, plus room to lift it out for cleaning. Keep it below the tank so it primes reliably, and keep connectors where you can see them. A hang-on-back needs clearance behind the tank. Rattling usually comes from trapped air, a dirty impeller, or the unit touching the cabinet; a soft mat underneath and a clean impeller often fix it. Use a drip loop on power cords so water cannot run along the cable to the outlet.

Common mistakes

• Trusting the box flow figure and ending up with weak circulation once media and hoses are in place.
• Pointing the outflow at one spot, leaving dead zones behind hardscape.
• Churning the surface in a CO2 tank, or keeping it perfectly flat in a heavily stocked tank.
• Washing biological media in tap water or replacing it all at once.
• Skipping an intake guard in a tank with shrimp or fry.
• Choosing a canister you cannot reach to clean, so maintenance keeps getting postponed.

Next steps

If your tank is new, read aquarium cycling and water testing before adding livestock. If you plan to inject CO2, read the CO2 system guide to coordinate flow and gas exchange. For an overview of the whole setup, see the beginner planted aquarium guide, and when you are ready to compare equipment, visit the filters hub or our canister filters for planted tanks, sorted by tank size. For two common canister lines side by side, see Fluval 07 series vs EHEIM classic and the Fluval 307 review.