sorry for the add but, there are areas were, you have probably seen these working waste water treatment plants that are totally biological with usuing multiple ponds as you mentioned the final ponds contain tilapia to eat the vegetation. and then sent to market, where people ingest them and take in the toxins from the fishes fatty tissues (which is where the animal kingdom stores toxins that it can not process) from the fishes sole diet of vegetation that fed off of sewage containing toxic chems and heavy metals.
that is my concern with closed system aquaponics where water containing trace elements that are at a safe level become concentrated because of evaporation replaced water and are taken up by the plants and obsorbed by the fish. anyone check their fish for what's in their tissues. every photo i've seen on this forum show fish living in water that resembles a sewer, yellow, brownish, cloudy.