Didn't seem to hamper their fight. The critters had one or two segmented "tails" trailing off of them, and they could be pulled off the fish, but there wasn't much wriggling or movement in them. They also seemed to be only on the back half of the fish--maybe less water resistance there. The areas where the parasites were attached were usually raised and either white or reddish, which would indicate an irritation of the skin/flesh. I'm sure that they are not completely harmless.