Abstract: 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.