I found fish? swimming in a barrel of rainwater. What are they called and how did they get in there? With winter coming, should I relocate them to a local lake or pond?

I live in Connecticut. A RubberMaid barrel sits below a carport awning and collect rain and runoff from the awning. The rainwater is used to water my garden. There is some dirt at the bottom of the barrel probably from submerging a dirty watering can into the water. There is also a fairly large rock at the bottom also to prevent an empty barrel from blowing away in the wind.

These fish(?) are about half the size of an inch worm and are kind of worm-like. Their head is slightly larger than their body. No extremities. They swim by wiggling from side to side. I don't believe they can survive outside of the water. When the water is moved and in motion, they curl up until the water is still again.