I work in the quarentine unit for the company Sealife, it's a company that owns around 20 public aquariums over Europe, they order items from us, we order them in from a supplier (GPOs come from Living Elements in Canada) and quarentine them for a set amount of time.
With things like Octos they are "Feature Creatures" so we usually have one of all displayed species with us at any one time as a back up for a worst case scenario.
Only got a Californian Long arm and a Small (Arm span about 120cm) GPO at the moment, getting O. vulgaris soon... lots of them and come currled!
We do get to see some fantastic things here, we kept Sea Snakes (Yellow Lipped Banded Sea Kraits) which I think are the most venomous in the world for a feature called "Venom" which I think is in Great Yarmouth this year. Every year the features pack up and move to another centre... very good idea!