It's not something you want to do often. Yolks are fattening, and they shouldn't eat too much of it. A little bit of it as a treat wouldn't make much difference.

What food items have you tried? Try silversides, guppies or rosy reds before giving up on them. Keep in mind, though, rosies aren't good for a long term diet, you'll want to try to get them to eat something else after they start eating.