I did some research of my own as I was not yet convinced it is fin rot. I just feed them live worm which could explain this that I found on a fish directory site. "Glowlight tetra - Hemigrammus erythrozonus: This gorgeous looking fish is a very pale, almost transparent, lemon colour with the internal organs clearly visible as a bright silver. A glowing gold/red line runs fron the eye to the tail and there is a red mark at the base of the dorsal and tail fins. When the fish are in peak condition, the tips of the fins develope a small white patch." If the fin starts to rip then I will be medicating.