DIY Radial Flow Filter for Aquaponics


Radial Flow Settlers are a type of filter that removes all the gunk and fish poop that occurs in a recirculated aquaponics system. Handy if you have overstocked your fish tank or over-fed the fish as the water quality suddenly gets murky and is less than pristine.


How does it work?

Fish effluent enters the center of the tank vertically and flows down through the stilling well, causing low-density solids to be easily settled out of the water column as they settle at the bottom of the tank.

Filtered water overflows a weir at the top of the settler into a collection trough where the clean water is sent off to the grow beds.

Commercial Radial Flow Settlers are expensive to buy.


Build your own Filter Cheaply

Thankfully you can build your own for under $100 if you’re reasonably handy and have some time to spare. You’ll need to find a good clean 200 litre blue barrel. For an average sized home system, the 200l blue barrel filter should work reasonably well. Look for the barrels on online auction sites as they sell for around $30-$40 or you can ask around shipping and storage companies in your local town.

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The flash animation on this page shows the principles involved. The central stilling well should be large enough for your hand to reach down unimpeded for maintenance.

You will need to have the stilling well centered in the barrel (not shown in the animation) with some stainless steel supports or timber rods to keep it level and in place.

Fish solids as they enter the barrel will swirl and drop down and eventually sink to the bottom of the barrel. Cleaner water taken from the top of the barrel will flow onto your growbeds or floating raft systems.

How efficient is it?

Radial Flow Settlers are said to be highly efficient. They wont remove all the suspended particles from the system but do a fine job in removing up to 48% of suspended solids and are said to be more efficient than a conventional swirl filter. You can read a scientific study on their usage here:

Bucket and funnel added to the system to capture the solids

Optional Extras

You get get super fancy and add to this system. Replace the central stilling well  with a bucket.

Replace the bucket’s base with a large funnel to capture and direct all the settled fish waste. Attaching the funnel to the bucket might be a challenge and perhaps some plastic welding or silicone will be needed.

You could experiment with an outlet pipe attached to the funnel and a suitable tap fitting to direct all that fish waste out from the system.

But should it be all removed and eliminated?

Why not direct all that wonderful nutrient to another tank that is not connected to the aquaponics system? Why not a wicking bed system to grow all your root crops?

Now that would make a great inventive addition.

Here’s a great little DIY Radial Flow Settler explained in this YouTube video.

Notice the little weir around the perimeter to further eliminate solids.


Removal of fish wastes is especially important in recirculating systems, as suspended solids can cause disease, damage gills, foul biofilters, increase oxygen demand, raise ammonia concentrations and cause off-flavor. The removal of suspended solids can be done by gravitational settling (sedimentation tanks, tube settlers, etc.) and physical separation (granular filters, screening, cartridge and bag filters), plus affinity bead filtration, vegetable filters and others.