My nana was looking after my mothers axolotl and I saw it wasn't being looked after properly it was just in a tank of water.... and that is it. So i took it and have done everything an axolotl needs to flourish and be happy. When I took him I noticed this big black lump that had grown on his side while in my nanas care, does anyone have any clue what it is and what I could do to make him better?