blanch some zuchini (courgette) just a small bit mind and put it in with young ones.
Another tactic is to hard boil an egg take a small amount of yolk and mash it up finely making sure the bits are really small, watch out though as it is high in protein so don't over feed with it or you will pollute your tank REALLY fast ! They may not like it either so be very cautious with how much you add.
Microworms are great. My fry would not eat anything but microworms. They are really easy to grow and they reporduce quickly. If you would liek more information e-mail me at [email protected]