Fish can be bad if you're a fan of biodiversity in lakes. Or at least it's bad to have fish in every lake.
Scientists have found that we're killing biodiversity with our love for fish and our penchant for plunking fish in every lake, pond, puddle, or stream. In Maine, for example, scientists believe that only about 50 lakes are fishless out of about 6,000 lakes.
What happens when we add fish? Various amphibians and invertebrates get eaten up and vanish. Swing by the Journal of the North American Benthological Society if you want to read more.