You're right. Advocating violence against someone who killed fish is going overboard.

The reason there is so much overreaction here isn't because some fish were killed. It is because so much is going into trying to do something good, and it all could go down the drain thanks to someone who thought pouring some bleach in a fish tank would be fun. It's not fun. This person may have killed the joy of children and adults alike. It's disgusting.