The white things are copepods, usually when they show up in large numbers it could be a sign that you are overfeeding. The copepods themselves aren't dangerous at all though, many fish love to eat them.

What are you feeding them? How much? Ich only shows up as white spots when it's actually on your fish, like a mosquito bite.

When they are free-swimming, they are completely invisible.
Also, don't necessarily believe what people in pet stores tell you. I have had several bad experiences with misinformed ignorant salespeople--keep in mind that their bottom line is to sell you products, not save your fish.