The Battle of Billy's Pond (1977)

When Billy takes home a dead fish he finds in a pond his cat pinches it and becomes ill after eating it. Billy and his friend Gobby set off to find out what's killing the fish and discover that industrial waste is being dumped into a stream that runs into the pond. After reporting the matter to the police they sneak into a detergent factory and videotape tanker drivers illegal activities but they're spotted and chased. Gobby's quickly caught and just as Billy gets cornered the police arrive.