Backyard Aquaponics
A place to discuss aquaponics

This may sound like the newbiest of newb questions but...
The fish poo makes nitrogen, but where does the phosphorus and potassium come from?

Same place as the nitrogen. Everything is not used by the fish and food that isn't eaten but breaks down in the system also contributes to these. You might also get some from the media you use and other additives like seasol or maxicrop which would add potassium among other things.

>> but where does the phosphorus and potassium come from?

everything has to be added - either via fish food, via water supply (not usually) or else directly

phosphorous is less of an issue, but usually trace elements have to be added.

we use seasol (seaweed extract) because it is low in nitrogen, if you have nitrates in the system you sometimes/often don't want to add any more. This adds most essentials. Iron and potassium are two you most commonly often have to supplement.

because of fish (sensitive to zinc and copper) and the presence of ammonia/nitrates already you have to be particular about what products/brands you use.