Gambusia affinis, an invasive species in California.

Unfortunately, the county distributes them to anyone who asks to release on their property.

They eat almost everything, including native invertebrates, fish, amphibians, and their eggs and young.
They can consume prey much larger than themselves, such as tree frogs by attacking in shoals and tearing off small chunks.

I noticed an increase in amphibian life after the fish were extirpated from a nearby pond.

I will be experimenting with Pimephales promelas as a means of mosquito control.

Last image: trap and bait used