Nile tilapia, Oreochromis niloticus, from a recirculation aquaculture venture in the Northeastern United States were presented to Cornell's Fish Disease Diagnostic Laboratory. The chief complaint was of chronic, low-level tilapia mortalities. Histopathological diagnosis of renal tissue revealed moderate to severe kidney disease due to the precipitation of calcium salts from the short-term use of calcium carbonate to maintain alkalinity in the production system water. Calcium carbonate had been substituted for sodium bicarbonate (the normal industry practice) due to economic considerations. A recommendation was made to return to the use of sodium bicarbonate for water quality maintenance. This change was instituted and levels of nephrocalcinosis in the aquaculture facility's Nile tilapia dropped significantly.