Abstract Methods to control the development of toxic by-products in the water were evaluated for various pet-fish species enclosed in polyethylene bags overlayed with oxygen. A formula of three components was developed that decreased the concentration of ammonia and carbon dioxide, controlled bacterial growth, and maintained a higher pH compared with untreated controls held under similar conditions. Clinoptilolite at 14 g/liter controlled the ammonia accumulation, tris buffer (0.017 M) at pH 8.0 controlled the accumulation of free CO2, and neomycin sulfate at 20 mg/liter prevented bacterial blooms. Fish were held up to 48 hours and at loading densities up to 180 g/liter with favorable results. Field tests of the formulation were conducted by air-transporting several species of pet fish from either Florida or east Asia to Seattle, Washington at loading densities that averaged 102 and 87 g/liter, respectively. The average transit time from Asia was longer than shipments from Florida, 36 hours compared to 22...