Try and introduce another food while feeding what they like. Just a little at first so they will take it. That way when you feed just that, they will be ready for it. Mine will not eat frozed krill. I give them a wide choice of foods and it is paying off. Nice color, nice size, and a great attitude.
Keep up those water changes while trying different foods. You need water changes all the time, but when I am giving new fish new food, I do WC everyday.