Two different size prototype aragonite reactors were tested to assess their ability to control alkalinity, dissolved carbon dioxide (dCO₂), and pH in marine recirculating aquaculture systems (RAS).  Control of CO₂, alkalinity and pH in RAS is critical in order to maintain health and maximize growth cultured organisms. The small-scale fluidized bed aragonite reactor was 2.5 cm in diameter and the large-scale reactor was a 0.6 m diameter Cyclobio^® fluidized bed.  The small-scale prototype fluidized bed reactor was filled with aragonite sand and tested under a variety of conditions manipulating alkalinity and dCO₂. Data from these tests were modeled using the relationship between alkalinity dCO₂, and pH across the reactor using multiple regression.    Subsequently the large-scale reactor was attached to an operating marine recirculating aquaculture system the reactor response of dCO₂, alkalinity and pH were observed over time to determine if equilibrium occurred.  The test with the small-scale reactor showed a change in dCO₂ across the reactor showing that an equilibrium alkalinity and pH could be achieved with the aragonite reactor.  This was verified during the tests with the large-scale reactor where equilibria were achieved within 4 days (alkalinity of approximately 135 mg/L as CaCO₃, dCO₂ of 9 mg/L, and pH of 7.0) that were stable during a 14 day test period, (Figure 1).  It is likely that fluidized bed aragonite reactor designs similar to those tested have the potential to simplify alkalinity and pH control, and aid in dCO₂ control in RAS design and operation.  Aragonite sand, purchased in bulk, is likely to be less expensive than sodium bicarbonate and could reduce overall operating costs during production.