My guess is that it is a table size aquarium with glass at the top. The water is filled past the level of the glass just enough to create that good roll. I've seen many coffee table aquariums that were similar. The way the Koi behave seems natural and contemporaneous and as if they are there rather than in another location with a camera feed. I raised Koi for many years.I don't know how that's done. Maybe a web search would give some answers.
I assumed it was some type of underneath projection mechanism. It doesn't seem like it would be a big tank of water. The surface seems to roll pretty much like a clothed table. Anybody know?