Black Spot

[scoobyandy]

I have a few zebra Danios in my tank and have all lived in there happily with no fatalities in almost three years. Recently one has developed a Black spot on his side. It is about 2mm in diameter. None of the others have it and I would like to know what it is and treat it/the tank before any others get it IF it can spread. Can anyone throw any light on what it may be and how to treat it.
Thanks

 

[WILDER]

Do you have any snails in your tank.
Take a look under black spot.
http/www.fish-disease.net/diseases.htm