marine recirculating aquacul-ture systems (RAS) in order to maintain health and maximize growth. A small-scale prototype aragonitesand filled fluidized bed reactor was tested under varying conditions of alkalinity and dCO2to developand model the response of dCO2across the reactor. A large-scale reactor was then incorporated into anoperating marine recirculating aquaculture system to observe the reactor as the system moved towardequilibrium. The relationship between alkalinity dCO2, and pH across the reactor are described by multi-ple regression equations. The change in dCO2across the small-scale reactor indicated a strong likelihoodthat an equilibrium alkalinity would be maintained by using a fluidized bed aragonite reactor. The large-scale reactor verified this observation and established equilibrium at an alkalinity of approximately135 mg/L as CaCO3, dCO2of 9 mg/L, and a pH of 7.0 within 4 days that was stable during a 14 day testperiod. The fluidized bed aragonite reactor has the potential to simplify alkalinity and pH control, and aidin dCO2control in RAS design and operation. Aragonite sand, purchased in bulk, is less expensive thansodium bicarbonate and could reduce overall operating production costs.