Aquaculture in closed pens or tanks is all about water management. As water is recycled, waste products will accumulate. This need to be controlled by designing systems with sufficient capacity to limit accumulation. In order to control this accumulation, it is of utmost importance to have a real-time monitoring system so that rapid changes in water quality can be monitored and action taken. In this way fatalities can be avoided and production optimised. Best financial results are achieved when the fish can live in a stable and healthy environment; clean water (few particles), enough O₂ and low and stable CO₂ levels without H₂S. Variations in water quality cause stress for the fish.