a system for nitrate removal from aquariums , both fresh water and marine aquariums , by means of permeable polymeric beads which contain a combination of fermentative and denitrifying bacteria and a carbon source . preferred beads are beads made from sodium alginate or chitosan . the bacteria , in the presence of the carbon source , are able to reduce nitrate to nitrogen gas . bacteria which are not harmful to fish are used . the porous beads used in the process are novel and part of the invention .