Based on only a very limited effort on my part, this was my experience as well. The fish prefer nitrogenous wastes to be as low as possible and pH in the range of 8, while the plants want copious nitrogen and require much lower pH for efficient uptake. I did not pursue the idea very far, however. I still wonder if a large bed of submerged-root plants could be utilized effectively to support a pond with no other bio-filtration present? The idea would be to simply have the plants scavenge most of the ammonia before bio-conversion (to nitrite) even took place.....just pondering......