You need to feed them all sorts of foods, not just flakes. I feed my guy mysis shrimp, brine shrimp, squid, and I put a clip of sea weed in twice a week. All of my foods are packed with vitamins!
Try some Garlic Guard and soak the food in it while defrosting it. It should present a better flavor for the fish, and it helps with other things.
Also make sure they have some caves they can hide in. My guy Loves his little cave, he sleeps in it every night. They also will turn pale in color when they are sleeping.
Fish Sticks, the best thing you can do with your dead fish.