Abstract: A floating hydroponic biofiltration device for use in a body of oxygenated water containing plant-eating fish, includes, in series, (a) a float and a terrestrial plant hydroponically mounted in a float aperture, the float aperture being essentially devoid of soil, (b) a relatively open mesh of substantial depth below the float, the mesh being open at the sides thereof to enable passage of oxygenated water from the body of water therethrough, and (c) a relatively tight matting defining tortuous vertically extending passageways therethrough for the plant roots, the matting anchoring the plant roots and at least partially excluding plant-eating fish from a portion of the plant roots passing therethrough. Preferably the device additionally includes, beneath the matting, a netting to exclude large plant-eating fish from approaching from below the bottom of the matting.