A swirl filter, sometimes called an aquaponics solids filter, is a device used to clear fish waste out of the water when that waste is too fine to strain out with a simple screen. Its job is to keep the water clean enough that your fish stay healthy. Rather than straining the water, a swirl filter spins it, and that spinning motion pulls solid waste out of suspension.

What Is a Swirl Filter and How Does It Work?

The idea behind a swirl filter is to slow the water down after it enters a large container, usually a barrel. The inlet pipe that carries water in from the fish tank is angled so the incoming flow swirls around the bottom of the container. As the water loses speed, particles denser than water begin dropping out and collecting on the floor of the barrel, where the accumulated solids can later be drained away.

In essence, a swirl filter works by using a rotating, slowing flow to concentrate solid waste at the base of the container. The spiral motion at the bottom generates a centrifugal effect that pulls solids together and keeps them from being carried back up into the grow bed.

Why Should You Use a Swirl Filter?

Before water heads back to the fish tank, it needs to be free of suspended matter such as uneaten food, floating debris, and fish waste. Some form of solids removal is essential if you want your system to run reliably. In an NFT setup specifically, filtering out solids also keeps plant roots from becoming clogged, which helps them take in oxygen and dissolved nutrients properly.

Just as a standard aquarium needs filtration, an aquaponics system benefits from a dedicated filter such as a swirl filter. It’s tempting to assume the plants alone will act as your filter and that no separate filtration is needed, and to a degree that’s true — worms living in the grow bed can break down organic matter in the fish waste and lighten the load on the rest of the system. Still, depending on your fish stocking levels, adding a swirl filter to further cut down on circulating solids is generally worthwhile.

If your water looks cloudier or hazier than it should, that’s a good sign it’s time to add a swirl filter. Beyond keeping the water clear, a solids filter like this can also let you expand your system into additional growing space, such as wicking beds or extra plant beds. You can even mineralize the collected solid waste through aeration and use it as a natural fertilizer for a soil garden. That mineralized water is notably nutrient-rich, so cycling it back into your aquaponics system is a cost-effective way to cut down on how much you spend on nutrients for the grow bed.

One alternative to a swirl filter is simply adding more grow bed space, though that requires room that not every grower has. If space is tight, installing a swirl filter lets you support more fish without losing filtration efficiency, since it does the job that additional grow beds would otherwise handle.

Bottom line: in a small DIY system, a dedicated solids filter usually isn’t necessary — worms in the grow bed will typically handle the solids on their own. If you’re running a grow bed with worms and still notice solids building up, it’s worth adding either another grow bed or a swirl filter. For larger or commercial-scale systems without grow beds, some form of solids filtration, such as a swirl filter, becomes necessary.

DIY 55-Gallon Barrel Swirl Filter

If you’d rather not buy a swirl filter for your backyard aquaculture or aquaponics setup, building one yourself is quick, simple, and inexpensive. Before diving into the build, it helps to clear up a common point of confusion: people often assume a swirl filter and a settling filter are the same thing, but they’re not quite identical. Settling filters tend to be larger and rotate the water more slowly than swirl filters. The differences between the two are fairly minor, and how well each one performs comes down mostly to its design.

When building your own swirl filter, pay close attention to retention time — how long a given volume of water stays inside the filter. For the best filtering performance, keep this retention time short, ideally just a few minutes.

Here’s how to build one:

1. Start with a large drum, about 55 gallons (roughly 200 liters) in size.
2. Mark two holes on adjacent sides of the drum, with one positioned a bit higher than the other.
3. Drill the two holes: the lower one becomes your inlet, and the higher one becomes the outlet that connects to your grow bed or biofilter. Use a proper adaptor at each connection so the fittings stay watertight and leak-free.
4. For removing solid waste, drill an additional hole at the very lowest point of the barrel. Fit it with a small half-inch PVC line, and add a valve so you can control the drain flow.

That’s really all there is to it — a simple, low-cost way to build your own swirl filter and keep your aquaponics system running efficiently.

DIY 5-Gallon Bucket Filter

This smaller version follows the same basic design as the 55-gallon barrel filter above, just scaled down. Here’s how to build it on your own:

1. Start with a 5-gallon bucket.
2. Get two adaptors of different sizes: one 3 inches and one 1 inch. The smaller adaptor marks the exit drain at the base of the bucket, and the larger one marks the inlet that connects to the grow bed.
3. Drill the holes you just marked, using adaptors at each connection to keep the fittings watertight.
4. For the solid waste drain, drill a hole at the lowest point of the bucket and fit it with a small PVC line and valve, just as with the barrel version, so you can control the flow.

Where Can You Buy a Swirl Filter?

You have two basic options: build your own filter, or purchase one from a local aquaculture or pond supply store. If you go the purchasing route, set a budget first and compare prices across a few different retailers so you can find good value without cutting corners on quality.

Searching online can help you locate nearby pond or aquaculture suppliers. It’s also worth reading reviews of different swirl filters before committing to a purchase. Avoid paying extra for features your system doesn’t actually need — stick to what fits your setup and shop accordingly. Take your time browsing online until you find a filter that meets your needs at a fair price.

Online forums dedicated to aquaponics and aquaculture are another good resource, since members often discuss the pros and cons of different filter products, including swirl filters. Joining one of these communities and talking with other members can give you a much clearer picture of which filter suits your situation and where to find it. You may even connect with experienced aquaculturists who can point you toward the right features for your particular setup.

Conclusion

A swirl filter is an effective way to remove waste from an aquaponics system, and it works especially well on larger particles that are denser than water and settle out easily. There are other approaches to reducing waste in your system as well — it’s worth experimenting to find what works best for your specific setup.

You can also cut down on how much waste your system produces in the first place. Avoid overfeeding your fish, since uneaten food both adds to the waste load in the tank and can clog grow beds, DWC systems, and NFT channels alike. Skimping on fish food quality to save money is also counterproductive, since lower-quality feed tends to generate more waste. Likewise, avoid overstocking your system — plan your fish quantities around their size once fully grown, since more fish means more waste, which puts extra strain on your plants and the system as a whole.

If you’re ever unsure whether your feed quality or filtration is adequate, check your plant roots. Roots that have turned brown or grey, or that feel slimy, are a clear sign that debris levels are too high, meaning it’s time to improve your feed quality and consider installing a swirl filter. Healthy roots, by contrast, should look clean and light in color.

If keeping maintenance costs down matters to you, installing a swirl filter is a smart, efficient investment for your aquaponics system.