While the facilities are well kept and interesting, the facility is small, and costs quite a lot. I understand this is due to expensive construction costs as it is on the roof of the building.
I can name at least 2 aquariums which are larger and also well kept: kasai aquarium and shinagawa aquarium. They are also more affordable. On the other hand, the sunshine aquarium is located within the shopping area, with great access to restaurants. 
So, I guess there is a reason people will keep visiting this aquarium
My son thoroughly enjoyed his trip here.