How high are the nitrates? Since this form of nitrogen is at the end of the nitrogen cycle, it isn't especially toxic to fish. It would have to be in the 80 ppm range and remain there for several days, before the fish would begin to feel the effects. Large, weekly water changes of at least half the tank's volume should maintain a steady water chemistry and this is all the fish require.
"The fish keeping hobby is very simple. Just change out a lot of water, a lot of the time!"