Cavities and Creatures

Why do dentist waiting areas always have a tropical fish tank? Must be the calming effect they have watching the fish, and in this case a shrimp like creature too.
Last week I was eating a sandwich, soft, nothing crunchy or hard, and I found myself biting on what felt like gravel. The enamel around an old filling had crumbled and broken into pieces. There wasn't any pain but I could feel the sharp edges where the filling had been.
I called the surgery and they fitted me in straight away but he only had time to do a temporary filling and today (Monday) I had a 40 minute appointment to dig out the old filling and put a new one in.
I took a few photographs of the fish and the prawn but unfortunately this is the only one that vaugely in focus.
I really don't mind visiting the dentist. I always think "At least I know the worst he can do whereas you never know what a doctor is going to find"!