Eel in my live rock?

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My live rock arrived yesterday.

Went down before bedtime to check it out, it was dark.

There was something fish-like swimming up and down very fast against the end glass. It was about 1-1/2 inches long, shaped kind of like an eel or tadpole, with a head area at one end. I didn't get much of a good view of it. As soon as I turned on the lights it zoomed into a small hole in the live rock and did not come out.

I don't know the color, it was in the dark.

I don't think it was a worm, just from the pattern of swimming, it was swimming the way a fish would.

Any ideas what this is? I don't imagine any type of eel or fish would survive transport in the live rock.