“...problem is that they don’t establish in pure sand coupled with our winters. Dilemma.”
My Mom had a pond with Koi and plants. Zone 5. She would keep the plants in her basement with some additional lighting in a big tub of water with a small circulating pump over winter. She’d give them a bit of liquid fertilizer a few times through the winter. She had good luck with that.
I don’t remember what she did with the fish; I don’t remember a big fish tank anywhere in the house. Maybe they just stayed under the frozen pond? I seem to remember her husband feeding them in the winter after breaking open an area of ice on top.
Hmmm, thought about building an offshore ‘wet’ nursery with a high fiber base to keep the water plants together and then transplanting the unit into the sandy pond. Where is an Apollo engineer when you need one?