For 30 years I have just put sandy soil from the garden, a little fertilizer tablet (from the aquatic garden store --Van Ness gardens)a layer of sandy soil and then put the plant/bulb/tuber in and then cover with more soil and then topped it with smooth stones around the plant, and place the planter inside a large, heavy pot. Our koi will nip the plant and tip it over without the stones and the extra pot.
when we took apart a 30 year old pond, we discovered half the plants were just growing along the bottom of the cement pond anyhow and werent in pots at all.