I have to disagree with Urbanfarmer on the subject of nitrates. There have been numerous studies on nitrate levels and how they impact fish. I actually remembered this from my biology classes as an undergrad, but I did a quick google search and came up with the following from several studies:

- Elevated nitrates above 100 ppm appear to substantially reduce antibody response in fish.
- Elevated nitrates above 200 ppm appear to cause damage to internal organs and particularly kidneys.
- Elevated nitrates above 100 ppm appear to negatively impact growth rate.
- Elevated nitrates at 200 ppm for 7 weeks tends to be fatal to fish.
- Elevated nitrates (no ppm given) tend to hamper oxygen absorption in fish.