How to edit a superflat world in minecraft?

Like I know there are presets but how do you add more layers? There's Grass on top, Dirt in the middle, and Bedrock on the bottom. I want to add even more layers so I can add an exhibit for water animals (I recently got Mo' Creatures) but some creatures can't be in 1-2 deep water levels… Help?

Guest

Hey you have to add the id's and depth in the seed. I'll link you a tutorial.

Fluhsay

I hate the slimes

Mista