Check out Diana Walstad's book The Ecology of the Planted Aquarium for some great info on natural, soil tanks. She's pretty amazing.
Bowls are the same as nanos. Nano simply means < 2.5 gal, IIRC. If you have enough plants, there's no need for mechanical filtration (this is species dependent, of course!!). Water changes and proper feeding, as well as proper fish for such a setup, make it pretty easy. I've done it with african dwarf frogs, shrimp, endlers, killis...I'm not saying it's the easiest thing in the world, and probably not for the "day one" hobbyist, but it's not difficult either. My adf tank was my favorite. My last frog lived five years in a nano...dang he was cute, too.