Bioball is an excellent biofiltration media, its only drawback being it takes up space. Bioballs are black plastic balls specially designed so as to provide lots of surfaces within the area they occupy.
It works great and are wonderful media for filtration especially in aquaponics NFT (nutrient film technique) and DWC (deep water culture) systems where proper mechanical and biological filtration prior to growing media is a mandatory process. The surface area of the bioballs have substantial value to micro flora mass.
