There are a lot of opinions on filter material. We will try and put some reasoning behind the choices.
Why do we use filter media? Two primary reasons. To reduce sludge and to convert the chemicals of the fish poo into nutrients for the plants.
Sludge reduces water flow in the pipes and system.
This is why we have one or two solids filters as the first filters coming out of the fish tank. We want the solids taken out first.
Some would say that the sludge makes a good fertilizer. That is true if the sludge has gone through a bio-degrading process to convert the sludge to something the plants can use. If the sludge is poured on or mixed into the soil it will enrich the soil, but only after the soil has broken it down to where the plants can use it. The bacteria in the soil will do that naturally.
The sludge has a tendency to build up. This build up can plug up a system's plumbing and bring the aquaponics system to a standstill. Sizing the plumbing correctly, greatly reduces sludge buildup. If the sludge is taken out in the first one or two filters which are placed close to the fish tank(s) and the plumbing is sized correctly so that they are encouraged to self clean due to adequate water volume, then the water carrying the nutrients will be able to deliver the nutrients to the plants most efficiently.
This unwanted sludge coats the plants roots, cutting off the plants from nutrients.
The plant's roots absorb the dissolved nutrients directly from the water. If the sludge is not taken out early on, over time, plant roots will be coated in sludge and the amount of nutrients available to the plants will be reduced. The plants absorb the nutrients directly from the water flowing over its roots. Sludge build up can also cause the sludge to decay in an anaerobic manner which will stink. A healthy aquaponics system will smell like a healthy garden, not smell like a sewer. If it smells like a sewer, you have neglected it and you have serious problems.
In a commercial aquaponics system, we recommend four filter tanks. Two will remove solids, then a bio-filter, then a degassing filter. We also add a UV clarifier after the degassing filter to reduce algae growth.
The first solids filter tank, in our system, is a cone bottom tank with a valve in the bottom. There is a slow circular movement to the water caused by an elbow in the in pipe just inside the tank. So the solids settle to the bottom and are regularly removed, before they start to decay.
The second filter tank is a mat type tank. We use 2 round Matala filters that are about 6 inches thick and the same diameter as the tank, stacked on each other. Another way is to use pieces of Japanese Filter Mat.
Some have tried to cut costs and buy regular filter material. While some have been successful, many found that all their fish died. Many types of filter material has fungicides and pesticides in them to keep things from growing in them. If these filter materials are used in an aquaponics system, you have just poisoned your system. Many of these types of filter media do not list these added ingredients. Some scouring pads that people buy for their kitchen are preloaded with soap. Soap will kill your fish.
Matala media is designed for aquaculture and pond filtration. There are four Matala products and each is a different color ranging from black green, blue, and gray. The black is pretty open and is too open for most aquaponic uses. We stack two 6 inch thick Matala round filters in our second filter tank. You will also need to regularly drain the solids from this second solids filter tank to keep the solids from starting to decay. Again these solids can be mixed in or placed on soil as a fertilizer with great success.