Did it just show up that size, or has it grown to that size..... like gotten bigger over time? Are there any others or just the one?
The reason I ask is because I was at
http://www.klsnet.com/files/fishchart.htm and I found this......
"Small white spots that get larger over time possibly with black streaks.
Fish Pox
No treatment. Keep up on water maintenance and symptoms should cease after about 10-12 weeks."
However, if you could get a picture it might help better to diagnose.