Aquaponics works by pairing fish and plants so each one solves the other’s problem. The fish produce waste, beneficial bacteria turn that waste into plant food, and the plants take up the nutrients while cleaning the water for the fish. A biofilter is the piece of equipment that keeps this cycle running smoothly so nothing in the system goes to waste.

Why use a biofilter?

A well-designed aquaponics setup usually runs itself with only light upkeep from you — feeding the fish and checking water parameters like pH, ammonia, and nitrate levels. So why bother adding a biofilter on top of that?

The short answer is surface area. The bacteria that convert toxic ammonia into nitrite, and then into plant-usable nitrate, need somewhere to live — and they colonize the surfaces of your grow media. If your grow bed doesn’t offer enough surface area, the bacteria colony will be too small to keep ammonia in check. A biofilter solves that by giving the bacteria extra real estate to multiply on, protecting your plants’ nutrient supply and your fish’s water quality at the same time.

In short, if your grow media can’t offer enough surface area on its own and there’s no room to expand it, a biofilter becomes a must-have rather than an option.

How does a biofilter for aquaponics work?

Think of a biofilter as bonus surface area bolted onto your system. Bacteria cling to the material inside the filter, and two separate bacterial groups do the work in sequence: one strain oxidizes ammonia into nitrite, and a second strain finishes the job by converting that nitrite into nitrate. Once these colonies are established, they largely take care of themselves and function like a natural filter, as long as you maintain good water conditions.

That said, some system types — deep water culture (DWC), nutrient film technique (NFT), and vertical setups — don’t have enough natural surface area to rely on this process alone, so a dedicated biofilter becomes necessary. You’re essentially giving the bacteria a home base; they still do the same job of turning fish waste into nutrients your plants can use.

When is a biofilter not needed?

Flood-and-drain designs, such as a chop-and-flip IBC setup, already have a built-in biofilter in the form of their grow media. If your system uses media like gravel, river rock, expanded clay, or lava rock — and there’s enough of it — you likely won’t need a separate biofilter. Systems without grow media, on the other hand, generally do need one.

Tip: Some growers claim a biofilter is optional on a floating raft (DWC) system because the dangling plant roots offer bacteria a place to attach. This only holds true at a very low fish stocking density, and growers who keep fish that sparse typically aren’t raising them to sell — they’re running what’s known as a low-density (LD) system.

Different kinds of biofilters

There’s more than one way to build a biofilter for an aquaponics system. Here are the main types you’ll come across.

Moving bed filter

Raft systems don’t offer much surface area for bacteria on their own, which is where a moving bed filter comes in. It typically uses small, circular pieces of media (often called K1 media) that maximize the surface area available for bacterial colonization. The media drifts along with the water beneath the rafts, purifying it as it moves.

Static filter

This style uses flat, tray-like inserts that slide into a container positioned ahead of the fish tank and before the plants, giving bacteria a large surface to grow on. You can choose media of different densities — denser media packs in more surface area and supports a bigger bacterial population. Place this filter after your solids-removal stage and before the water reaches your plants.

Drip filter

A drip filter lets water trickle in from above, passing down through a chamber packed with a high-surface-area medium such as lava rock or oyster shell. As with the static filter, it should sit after your solids filter and before the plant beds. As water works its way through, the ammonia is converted into nitrate that’s ready to feed your plants.

Important factors to consider

Before adding a biofilter to your setup, keep these points in mind.

Remove solids first

Biofilters aren’t built to process large clumps of solid fish waste. While a full pipe blockage is uncommon, a bigger concern is solid waste building up inside the filter itself. That buildup creates low-oxygen pockets that attract the wrong kind of bacteria and shrink the effective surface area of your filter, which lowers its efficiency and can let ammonia levels creep up — bad news for your plants. Installing a dedicated solids filter ahead of the biofilter, and emptying it regularly, prevents this problem.

Bacteria need air

In a typical aquaponics setup, the bacteria get regular exposure to air — either through the draining cycle of a flood-and-drain bed or because part of the grow media always sits above the waterline. Since a biofilter is a closed, self-contained unit, it can accidentally cut the bacteria off from that airflow. Make sure your biofilter has its own air supply so the bacteria can keep functioning properly.

Low-density vs. high-density systems

If you’re running a low-density stocking rate, you may be able to skip the biofilter altogether. As a rough guideline, that means staying under about 0.3 pounds of fish per square foot of grow area — roughly 300 pounds of fish for every 1,000 square feet of growing space. At that low a stocking rate, the fish simply don’t generate enough waste to require a biofilter, since the exposed roots in your grow beds provide sufficient surface area to handle the ammonia. Just keep in mind this assumes you aren’t overfeeding your fish.