AbstractThe Seattle Aquarium has reared coho and Chinook fingerlings since 1978 in a 36,000 liter recirculating freshwater system utilizing clinoptilolite (clino) for ammonia removal. The system has effectively produced 140,000 smolts, due in large part to the efficient removal of fish‐excreted ammonia by the clino filters. Quantitative studies were performed to assess clino filter loading capacity and to establish effective recharging techniques. The operational characteristics of the clino filters and suggestions for using them in freshwater recirculating culture systems are discusse