I wouldn't bother with the lettuce or zucchini, corys are omnivores.
As Cooper2085 said Hikari First Bites and crushed sinking tablets. You can also feed them micro worms, hard boiled egg yolk, and finely crushed flake food.
With the finely crushed food (and the First Bites) I soak the food in a tiny container of water for several minutes before I feed them so the food sinks faster.
Cheers.