Puffer fish has weird mark
2 of my figure 8 puffer fish have got a weird mark at the base of the tail just before their tail fin. It is red and like a thin thread like mark that sits below the skin. The puffer fish are acting normally but this mark is present on both fish in a similar location. Both fish have eaten within the last 24 hours. the other figure 8 puffer has no mark so far. Water tested was well within parameters.
wondered if it was a parasite but having read on websites cant find anything that matches exactly.