I have had large, medium and small ponds with goldfish wcmm, livebearers and killies, with and without plant (not all in the same pond). It works very well with no filters or pumps. The oxygen level will improve with good surface area. Down here, in the real world, it pays to have an area of reasonable depth that wont freeze in the winter. I have a friend with wcmm I gave him to keep down the mossies that has 2 x half barrels and a smaller pond very heavily planted with lillies (leaves all over the surface) and the white clouds are happy and breeding like mad at the moment. Ignore the pet shops, they just want to sell you stuff.