Some people use top soil. The substrate basically depends on the type of plants you have. Finer soil for smaller plants with shorter roots. Loose soil for larger plants with longer roots don't have a hard time growing out.
Aside from the substrate, you might want to look into the types of plants you want to grow - make sure you only keep plants with similar pH, temp, lighting requirements. They're going to be in the same tank after all.
Don't touch the fish. Guard Bunny is watching. Bunny no like.