Mike is definitely onto something. Phytoremediation is highly effective and through bio degradation won't turn wetlands into an EPA superfund site unless you are capturing heavy metals or PCB's.
I would think your next biggest water quality issue is the high protein pellets used to feed fish. Hypothetically, if you are feeding fish at a rate of 2.5% of body weight, increasing the rate to 2.8% would increase the organic nitrogen load in your pond sediment by 20%.
Like most pets, people tend to overfeed them and I doubt pet fish are any different.
