I had a worm in my aquarium similar to that. It didn't bother anything. It was really fast I use to catch glimpses of it here and there, but I always thought it was cool that whenever I sat down to gaze into my aquarium that there was a possibility that I would see it. It was about six or seven inches long. I enjoyed it and thought it was awesome, but one day my wife caught my red coral banded shrimp with it in its claws as he was eating it.