pods don't kill fish, however the number they are in. I have tons of them in my tank and the only dangerous are the large amphipods and they are only dangerous to zoanthids.

Are copepods harmful to fish?

Copepods are an incredibly nutritious food source, so it's really great if your aquarium animals consume them. However, pressure from predation may be strong enough to limit the population size in your system.

What do copepods do to fish?

Ecologically the planktonic copepods provide functionally important links in the aquatic food chain feeding on the microscopic algal cells of the phytoplankton and, in turn, being eaten by juvenile fish and other planktivores, including some whales.