You can loose any fish to disease, I know damsels are tougher, but by in large they all get meaner when they get bigger, except for maybe the green chromis. You need to find out why you have a problem with looseing fish, are you useing a Q-tank, and are you buying healthy fish form a good lfs? Are you adding your fish too quickly, Alot of people make that mistake, I know I did. Good Luck