Some research has suggested that with enough water flow the plants will still be able to absorb the required nutrients even if the concentrations are lower than what is commonly suggested. That is why previously I had suggested to have constant water flow in the grow bed (in any form, just don't stop it). If you find a better answer to your question, please share. However, I don't think aquaponics has enough scientific research (or specifically applied therein) to have a conclusive answer yet.