Firstly fancy and comet goldfish shouldn't be housed together as your comets will out-compete your fancy goldfish for food, and prefer a cooler water temperature than fancy goldfish do.
Secondly, comets really shouldn't be housed in anything less than 100 gallons long-term. They can grow over a foot in length, and a pond is probably the most ideal home for a comet or common goldfish.
Thirdly, can you give us some more information. Tank size, filter, how long has the tank been set-up, what are your ammonia, nitrite and nitrate readings? Things like that will help to determine the exact cause of your problem.