Good use of negative space, too. I like the bubbles around him and the angle you took it at. I wish there was something else in the shot to provide context, but this is good work.
Nicely taken and good composition. Oddly enough, usually when I see an aquarium photo I feel it should be more like this (no sign of the aquarium surface itself, no reflections, no kids standing infront of the glass) but in this case, in this challenge I think it might be even better with a small child with their back to the camera to make it clearer the animal is looking out, or perhaps another visual clue that this is a confined space. That said, there are clues there, such as the clean curve of the tank surface so... it's not like it's not possible to figure out what one is looking at.
Anyway, I think this is a really strong entry - best of luck!