The depends on what youre wanted them to clean, left over food in the tank, algea?
Peppered corys are my one of my fav fish and for left over food theyre the best with huge character. 20g minimum, though i would say bigger if possible.
For Algea, i would recommend amona shrimp as eat almost all types of algea. Any size tank really.
Fish die belly upward and rise to the surface. It's their way of falling.