just to let you know, pristellas are a type of tetra that is scaleless, what more than likely happened is he scaped the slime coating off in that area and damaged the skin abit. just keep a close eye on him but should be ok. pristellas are very hearty little fish, I have 6 that have been through ammonia poisoning , ICH, and when I moved the contents of my loaner tank to my new 55 gallon one of the pristellas got stuck in the net and was out of the water for about 15 minutes until I noticed it. I thought he was gone, but by morning he was swimming in the shoal with all of the oters and has made a full recovery not more than 3 weeks later.