the werid part is that I don't got any snail in my tank. so I don't know how can they reproduce or something. so if I treat the skin will that black spot heal up?[/QUOTE]
Hello; A possible answer is that the fish had the parasite before you got them. Are the fish new? That you do not have any snails should mean the parasite cannot continue it's life cycle in your setup.
I went back into the thread and do not find much information. Are the fish in an outdoor pond? What general part of the world are you in?
My take is the parasite matures in the fish and then makes it's way out leaving a hole. That hole can become infected? Read RD's post again as he suggests treatment.
The stage of the parasite that left the fish looks for a snail to inhabit, so no snails then my assumption is the parasite will eventually perish. My additional take is you should be careful with the equipment used around the infected fish.
I am making a big guessing here but it might be wise to somehow do something about the water from the frequent water changes (WC). Again it may depend on where you are. I may be overthinking this but if you do some frequent WC there may be some active parasites in the throw away water. If you live where the parasite is endemic then not necessarily a big deal. However if you got the fish imported to you and the parasite in not native then dumping the waste water directly into the environment might possibly open the opportunity for the parasite to become established locally.
I never studied this particular parasite but it's life cycle seems similar to one that infects people. Some parasites have a need for very specific hosts and so are not able to spread where these particular hosts do not exist.
