Vetts, can you give us a little more information? Have you seen something, or do you simply suspect there are parasites? if you can see something, can you describe it? In the water or on the fish? A growth or an actual living organism attached to the fish? Color? Shape? Size?
I'm sure that, with enough info, someone here will be able to diagnose for you.
Regarding food, yes, they do sell special "winter" and "summer" feed formulas. The summer food has less carbohydrate content, and us used when the water warms up.