One of the beautiful things about the conjoined practice of aquaculture and hydroponics is that each complements the other, creating a more complete version of the natural cycle of nutrients. The combined setup allows fish and plants to establish a symbiotic relationship (still aided by humanity, but less so) that eliminates many of the disadvantages of either aquaculture or hydroponics alone. For example, in a functioning aquaponic system, growing plants are able to absorb nutrients in the water and maintain a well-balanced water chemistry. When the water is clean and healthy, there is no reason to do any water changes. In fact, the only use for additional water is generally to top off any that has been lost to evaporation.
In more traditional aquaculture systems, a high population of fish coupled with a lack of plants means that, as the fish eat and produce waste, bacteria go to work decomposing both the waste and any uneaten food. If the byproducts of that decomposition are not used up (by plants, for example), toxins will build up, the water quality will suffer, and the fish may eventually die. In order to mitigate the buildup of toxins and algae, Operators of traditional aquaculture systems must perform regular water changes. Hydroponics operators, on the other hand, must add fertilizer (usually chemical) directly to the water in which the plants are growing. As you can see, Aquaponics permits the two systems to perform better in combination than either can manage alone.
