The nutrient content (nitrogen and phosphorus, N and P) of the dry weight gain of fish relative to N and P content of the dry weight of feed was used to determine nutrient retention in five species of fish that were reared in a commercial recirculating aquaculture facility. The culture system had five 39.2 m3 dual-drain culture tanks, one tank each with largemouth bass (Micropterus salmoides), hybrid striped bass (aka sunshine bass, Morone chrysops x Morone saxatilis), and rainbow trout (Oncorhyncus mykiss), and two tanks with walleye (Sander vitreus). All fish were exposed to the same water temperature (15.8 - 24.1°C) and water quality. On the first day of the study, most rainbow trout (643 g) and walleye (497 g and 398 g) were at or near market size, whereas the largemouth bass (73 g) and hybrid striped bass (96 g) were fingerlings. Measured for a 56-d interval, the range in nutrient retention was 12.0 to 44.1% for N, and 14.8 to 53.8% for P. Nutrient retention was related to fish species and size; e.g., the larger size-group of walleye had nearly half the retention rates of the smaller size-group of walleye. Highly significant (p ≤ 0.01) positive correlations occurred between retention of N and P, protein efficiency ratio, and net protein utilization, but nutrient retention was inversely related to food conversion ratio. Total ammonia nitrogen (g kg-1 feed fed) in the culture tank was inversely related to nitrogen retention. Values for TAN production ranged from 2.9 to 6.9% of daily feeding rate. This study demonstrated an interaction between nutrient retention with fish species, age or size, growth rates, temperature, feeding rates, nutrient content of the feed, and protein retention, all of which are factors that influence biofilter capacity to handle ammonia production and unit processes to reduce N and P content in the effluent.