Siberian sturgeon (Acipenser baerii) is the most commonly available sturgeon in the UK. Its plain brown/black colour when small shouldn’t be frowned upon as they will make up for it in later life when they are large and feed out of your hand! One of our customers purchased one last year at over 4ft and it follows him around the pond like a dog! (his words not mine). Due to their size large, well filtered and oxygenated ponds are a must.
Siberian Sturgeon will not eat your other fish. They are not suitable for aquariums.