Avoid feeding more, as you will add unnecessary ammonia to your tank. Instead, examine their feces. Is it stringy and sort of white or clear? If so, then you've probably got parasites. Go ahead and pick up from your LFS some medicine for internal parasites. I've never had to deal with this, so I'm not sure of what brand to suggest. However, continue to feed your normal amount, watch any other fish in the tank, and make sure that they eat everything you drop.