Tank Parameters: PH 7.0, Temperature 76F, Ammonia 0, Nitrite 0, Nitrate >10PPM I made a similar post a few days ago, but I really haven't come up with a conclusion as to what this could be. I originally purchased 4 danios, 1 of which was acting odd off the bat and died overnight. The 3 other danios seemed healthy, however one had a weird mark on his dorsal fin. The one with the weird mark ended up acting weird by day 4, and by morning of day 5 he was dead. I now have 2 danios left, both are acting perfectly fine, eating happily, being very active. However I've noticed white marks appearing on one of the danios. I'm not sure if these are scratches, or possibly some kind of fungus? I've been treating with melafix and primafix for about 4 days now (Started treating when I noticed the white marks). They definitely aren't going away and another white mark popped up on him today (or at least I noticed it today) Click image for fullsize, mark on the left is the new mark, mark on right is the old one. I'm honestly not sure if this is some kind of fungus, or maybe he's scratching himself accidentally on something inside the tank, clueless at this point. I want to add more danios, so they can actually have a complete school, but I'm not comfortable adding more fish when I don't know where the these marks are coming from.