i can see having this used in a well designed indoors pond.

Yeah, that fish loft is not a great idea. Coi and goldfish need more lateral movement than vertical. And they’re not the brightest animals - they could very likely get confused and “lost.” Besides, the temperature and pressure differece that’s bound to occur between the regular pond and the loft could cause problems for your fish. It’s another example of form not follwoing function - unless, f course, the function is to look good for a party and have all the fish die the next day. I wouldn’t risk it.