Since your system will be depending on a natural balance of nutrients through fish, bacteria, and your plants, it's typically unnecessary to ever completely replace the water in your aquaponics tanks. In fact, it's reccomended to never replace more than 10% of the water in your system without thorough testing and treatment. This balance will prefer gradual changes, as any major swings in temperature, or concentrations of nutrients or chemicals, can be harmful or even deadly to fish and plants alike.
Water from your system will be lost overtime through evapotranspiration, and it may be necessary to refill it every now and then. Be sure to be adding treated, high quality water, and to do it in slow steps. Major swings can be fatal, especially to fish, so it's typically suggested to add water through the grow bed, rather than directly into the fish tank. If you test your water and find something wrong, be sure to add solutions in small doses in order not to shock any of your residents.
