Hi,
Are they black spots? If yes read this:
Symptoms:
The most obvious sign is the small black spots, measuring approx. 2mm in diameter (.08 in). Fish may also exhibit behavioral signs, such as scratching against aquarium objects and flicking of their fins.
Cure:
Black Spot is fairly easy to cure. First and foremost, remove all snails from the aquarium. And that's about it! The parasites can not mature further so eventually die. If any of the sites of attachment get infected by bacteria, use a mild antibiotic available from your local aquarium store. This rarely happens however, so is not usually necessary.
Dave