Never mind, I found out what it is. Bob Fenner from Wetwebmedia.com told me with 100% certainty that the dartfish got too close to a bristleworm and got stuck. He said that they will fall out after awhile.
Yeah, Bob Fenner said, "Ouch, I do know what those are. There is no doubt, your dartfish got a little to close to a bristleworm." He just keeps swimming around, he doesn't seem to be affected by them.