Here are just a few examples of negative effects the introduction of non-natived species that had occured in the past.
The Nile Perch, Lates niloticuswas introduced as a game fish into Lake Victoria, and since its introduction has elmiated over 90% of all the lake's native cichlid species. Species are now extinct, due to its introduction.
Tilapia rendalli introduced into Australian water,
Has driven over 20 species of native fish into extinction, and dozens others are threatened due to not being able to compete with the Tilapia for food, and also from predation by it
A species of snakhead were introduced into a couple lakes in the US (maryland if I remember correctly) a few years ago, and in the few months until they were captured, they had consumed literally thousands of small native fish, if they werent captured, they would have seriously threatened dozens of species. This has also lead to an outright ban on this species in the US.