^ This.
The town next to mine has notifications warning in public buildings of the 15ppm Nitrate value of the untreated tap water. I have a nitrate reading from my well that ranges between 7 - 10 PPM right out of the tap!
The good thing about this is:
1. No Chlorine whatsoever in my well water!
2. Next to chlorine in generally utilized aquarium refill water Ammonia is the following most lethal water parameter for most of the fish we keep. We are no worse off than anyone else here!
3. Nitrites follow Ammonia as the next most lethal water parameters to the fish we keep. Again we are in the same situation with Nitrites as anyone with 0 nitrates from their taps.
4. The forty PPM is indeed a good "ideal" maximum value for nitrates. This is however highly relative to the fish species, and is a average value set closer toward the lower end of the general hardiness of the normally kept fish species.
5. Most fish that have a minor degree of hardiness can be acclimated slowly to higher levels of nitrates far in excess of 40ppm.
With this said, it is important to adapt your fish-keeping to species that are tolerant under the water conditions that you can most easily supply for water changes.
If you still feel the need to keep fish, that I would NEVER consider keeping with my slightly elevated nitrate levels from my well (Like the beautiful discus cichlids), then you will most likely have great expense and require an alternate water source.
Another option would be to find a full sized home water filter system that reduces the nitrates to <10 PPM levels.
