I'm a beginner with fish tanks. I have a 10 gallon tank with 3 fish (2 guppies and 1 platy). I am thinking of getting new fish (my platy is lonely). Recently my guppy has developed a large lump right on top of the tail bone. The lump is a plastic-like orange lump. It has white stuff growing on top. I am extremely worried about him, and I would appreciate if you helped me identify the cause. He acts and eats like a normal fish. None of my other two fish have had any problem with this. My theory is he got injured and the wound was infected with some sort of bacteria or fungus. The lump looks like it's sucking on him. I'm very worried! Thanks!