The first thing you should always do when any of your fish gets sick is make sure all your water conditions are in excellent shape. Do you have a liquid test kit? If so do an ammonia, nitrite, and nitrate test. Ammonia should always be at 0, nitrite 0, as well, and your nitrate below 20, but preferably below 10. If you do not have a water testing kit, you really need to buy one, and the liquid work best, the strips are inaccurate. If you don't have one though, begin by doing a 50% water change, to ensure all medicine is out of the tank, and the water conditions are good.

For further treatment you should get the medicines primafix and melafix. Treat until you see an improvement (this generally takes about a week). Do a 50% water change once a week until the fungus is gone, and be sure to invest in a liquid test kit. API master test kit is one of the most affordable and best working.

Best of luck, and let us know how things are going. =]

Also, please remember that all pet stores, fish or otherwise, are just there to make money. They often give very poor advice, and lead hobbyists down the wrong path when it comes to proper keeping.