A method for reducing ammonia concentration in an aqueous medium caused by the presence of fish gill/urine discharge, fish faeces and uneaten organic fish-feed particulate matter in recirculating aquaculture tanks. Fish in fish tank water are remotely located or partially isolated from the fish tank water utilized by the sono-molecular-conversion apparatus. Transient cavitation-created, collapsing microsized bubbles generated in the fishless portion of the fish tank water completes ammonia nitrification, mineralization and denitrification therein.