Dont know about discus fry, but for my natives (small bullies and whitebait) I do this:
Steep 1, slice heart thinly
Steep 2, cut the thin 'steak' into thin strips
Steep 3, cut the strips into mini cubes
Steep 4, feed to fish
Done with frozen heart and a very sharp knife.
Steeps 1-3 can be done with the wife's 'fine' cheese greater (if she isn't around to see)
PS I did try in the food processor :sick: but was too fine for my larger fish