a growing fish population , started in a single tank is sub - divided as the fish grow so that the capacity of the fish to absorb nutrients and produce waste , throughout the growth process , is matched to the capacity of the container in which they are housed . each such container is essentially an independent closed loop system with a relatively fixed capacity to aerate heat and filter water as water is recirculated . preferably several populations are cultured simultaneous , with each at a different stage in the growth process . in this manner relatively constant and efficient use is made of all tanks at all times and the fish harvest is semi - continuous .