Where does the Phosphurus comes from??

[Felipe Cohen]

Plants need Nitogen (N), Phosphurus (P) and Potassium (K) for growing. As far as I know, Nitrogen is avaiable, in a long-term aquaponic system, as Nitrate (biological filtration), that is constantly added from fish excretion and food degradation. My recent concern is where does the Phosphurus and the Potassium comes from?? Phosphurus is avaiable as Phosphate. However in a long-term aquaponic system, this element will be absorbed by the plants. I know that this elements are avaible in the fish food, but would that be enough?

Should I add any compound? 

[TCLynx]

Yes, the fish food and fish waste generally provides plenty of Phosphorus so there is no worry on that one.  Now the potassium is often another story and in many systems it does get supplemented along with trace elements by the use of seaweed extract or in other systems people might use potassium bicarbonate as a pH buffer that also provides potassium while other systems might use Potassium hydroxide alternating with calcium hydroxide but they are strong and must be handled carefully.

[Nate Storey]

Hi Felipe,
No, you should not have to add phosphate to your system. Phosphate is available in the fish feed and is really, really water soluble, so it is quickly incorporated into your biological cycles in-system. It's a plentiful compound in your feed in the form of both plant and animal matter (cell membranes, lipids, DNA, etc.) and you shouldn't have deficiencies- unless you are discharging large amounts of water. . . Potassium on the other hand can precipitate out if you're using too much hydrated lime or if your water is exceptionally hard.