Water quality criteria for aquaculture systems have typically considered parameters such as temperature, dissolved oxygen, total gas pressure, ammonia, and nitrite. Many of the published criteria are derived for environmental protection of a wide range of species and life stages. These criteria may not be appropriate for a single species and life stage, especially in commercial applications. The value of a given water quality criterion may depend strongly on the species, size, and culture objectives. In water reuse systems, fine solids, refractory organics, surface active compounds, metals, and nitrate may become important. The limiting factors in very high intensity reuse systems are not entirely understood at this time. Development of more relevant water quality criteria for reuse systems will require production-scale trials. Separate water quality criteria for biofilter operation are also needed.