The use of polycaprolactone (PCL) for removal or reduction of inorganic nitrogen compounds, especially nitrates, from biological holding waters, e.g. aquariums, in which the removal efficiency depends on the grain size of the gravel filter material. Independent claims are included for: (1) A means for removing or reduction of inorganic nitrogen compounds, especially nitrates, from biological holding waters containing a biologically degradable polymer, especially PCL and (2) a fleece filter containing PCL granulate.
