Having black walnut near (literally touching) our pond hasn't caused any appreciable damage to the fish or pond. This pond has been there for 30 years and there are 5 or 6 black walnut trees within 10 feet of the shore and walnuts do fall into the water. We've always had good fishing.
They do cause some problems near a garden but it's debatable on whether that is due to any poison produced or simple water uptake from a large tree. We had one next to the garden and everything near it was slow to grow but the tree was also very large. We cut it for lumber finally since it was worth quite a bit. It's better to harvest them than let a storm bring it down.
