I recently found out that a large koi angelfish does wonders on guppy fry. He has eaten 20+ fry and 2 adult female guppies. The adults were not intentional, I went away for a weekend and he got hungry.
kribensis12 is right about dropping them in the tank. If I net some fry from my 10g and throw them into my 30g it turns into a feeding frenzy. Diamond tetras also gobble up fry quickly.