Data collected from a single grow-out tank in an abalone farm in southern New Zealand has highlighted hygiene maintenance problems in the use of semi-closed water conditioning systems for the aquaculture of New Zealand black foot abalone Haliotis iris. The data shows that semi-closed systems can have high concentrations of un-ionized ammonia, which is harmful to the animals. In this paper an alternative open flow-through system is suggested where energy demand is limited by heat recovery at the grow-out tank outlet. Using temperature data collected over 1 year, and a previously obtained expression for standing losses, a simple energy model is presented for an open system with heat recovery. To compliment the energy model, a function has been established for abalone production with respect to the concentration of un-ionized ammonia and water temperature. The energy model and production function are combined to determine the impact of plant design and tank conditions on the economics of the operation for the southern New Zealand climate. It is demonstrated that temperature control is financially preferable to an open system with no temperature control, and estimates of optimum operating conditions are given. Copyright (C) 2004 John Wiley Sons, Ltd.