Aquaponics is the combination of fish production (aquaculture) with the soil-less production of plants (hydroponics). It operates within a closed-loop system and utilizes minimal resources. Fish feed provides most of the nutrients required for healthy plant growth. These nutrients, excreted directly by the fish, or generated by the microbial breakdown of organic wastes, are absorbed by growing plants. Nutrient removal by the plants in turn treats the water by removing nitrogenous compounds, such as dis-solved ammonia, which are harmful to fish. Water is then re-oxygenated and returned to the fish tanks. Fresh, potable water is added to the system, as necessary to replace evaporative loss.