Tom La Bron wrote:

You are also right in saying the food has nothing to do with nitrates.

I'm confused. Doesn't "high quality" usually mean "high protein"?
Isn't protein high in nitrogen? I know that the manure collectors in
olden China would get better prices for human shit collected from the
rich part of town because it made better fertilizer. Seems to me that
feeding better food would actually increase the pollution problem,
even if the quantity was reduced, due to the fish having such a short
intestine (low food conversion efficiency).

Of course, a "lower" quality food would have a higher roughage content
and produce a different problem to be cleaned up. OTOH, bacteria
attacking the undigested roughage should bind nitrogen, and reduce the
nitrate problem, wouldn't it?