Well, the new diet will definitely turn it around. Just feeding flake food could have caused this.
I would pick up some vitamin supplements that are high in vitamin C such as Kent Marine's Vitaminc C, VitaChem, and Selcon. All of these things dosed on the fish's food will greatly help.
Also, I am not a fan of the lettuce clip they give you. Here is what works wondefully for me. Break off a piece of the seaweed, put it into a small cup of tank water, put a few drops of your vitamin, let it soak for 10 minutes, and then rubber band the piece of sea veggies to a piece of rubble rock. Putting it on the rubble rock is a lot more natural for the fish. They have never seen a lettuce clip in the wild, so this is very unnatural to them at first. The rubble rock is more natural because then the sea veggies is like a piece of algae growing on the rock.