Hi, I've noticed a few of our lemon tetra's have these grey/black marks growing on the base of their tail fins. We tried dosing with an anti-Internal bacteria product that seems to have made no difference. The effected fish are still swimming around and eating, however their colouring is now a little dull compared to their tank mates. Could it be old age? Some of them we have had for 18months now (now sure how long Lemon Tetra's live for). Does anyone know what it could be or how to treat it.