Hydroponic systems rely on the careful application of man-made nutrients for the optimum growth of plants. The nutrients are made from mixing together a concoction of chemicals and trace elements to form the 'perfect' balance. Water in hydroponic systems needs to be discharged on occasion as the salts and chemicals build up in the water which can become toxic to the plants.
Aquaculture systems focus on maximizing growth of fish in tank or pond culture. The fish are usually heavily stocked in the tanks, often 10kg in 100L of water. The high stocking rates often mean that the tank water becomes polluted with fish effluent which gives off high concentrations of ammonia. Water has to be discharged at a rate of 10-20% of the total volume in the tank on a regular basis. This water is often dumped and may pollute sensitive ecosystems and destroy waterways.