Here is a very good article on water quality from Aquanic. It points out fish waste as a problem. Especially the higher the fish density. It is not so much the artificial feed which may be left over but that which is eaten , used and excreted (as in more lbs. of fish, no matter the source of food, and thus more waste and more demand by more Lbs of fish to support). I think your water quality point is well made and note there are many aspects to water quality.

http://aquanic.org/publicat/state/il-in/as-503.htm