The above fish tank is not unique. It is apparently a common craft. Recyclart rounded up photos of several old pianos turned into aquariums. Some of them, such as the one in the video below, remain playable as electric pianos. You can read about one man's experience building one here, which includes this inevitable story:
The kids in the neighborhood would come visit me to see the work in progress. The typical response was, "Cool, but why?"
Some people will never understand.