Charles seems to be right–my boyfriend (who, conveniently, is Japanese and translated this video clip for me) says that the fish are just to demonstrate that there really is water under the fryer. The point of the video is that by having water underneath, the extra bits of batter from the croquettes fall down into the water instead of staying and burning in the oil (keeping the oil clean), and the danger of a water/oil explosion is reduced because the water sinks down before it can explode. (They actually never say anything about the goldfish eating the crumbs…)
Interesting video though!
