Pond only. None fit in tanks. Even youngsters needing to be grown on would need a chiller (£300+) to keep them cool enough!
Cant go in ponds effected by blanket weed either as they cant swim backwards and get stuck and die...
Minimum for a Sterlet is about 3ft (90cm)... minimum for sturgeon is more like 4-5ft (120cm-150cm).
Pond will need to be minimum of 5ft deep, spot on water quality and good filtration and a great big UV steralizer to help combat blanket weed.
Very few species available to be honest.