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.
I can back up wmfamily's above statement by saying my pond also has black walnuts close-by and up hill from the water. Nuts fall into the pond and the water has actually been tee colored presumably from watershed from the walnut and shagbark hickory groves. My fish have only been in the pond for about a year and no signs of poisoning (prior to refurbishing the `50 year old pond sustained GSF and crawdads with out issues).
I have read rather convincing articles about the black walnuts ability to act as a poison, but wm's and my situations must be below the threshold.
My two cents