Try plant matter (make sure they're not toxic), algae (where did you get them from, there maybe algae there or algae disks or green water), freeze (or boil) some organic lettuce (no insecticides).
Feed in small amounts but often.
After my green frog & toad tadpoles @ 3/8" (w/o tail) started eating dry dog food and grew very well, last ones are just leaving the tank.
Spirulina pellets are good. Yank some algified rocks out of a tank or pond and let them feed on that. Just don't let blanched foods/dry foods sit for too long or they'll start to rot.