Tropical Fish Symphony

Pretty easy to do with off the shelf stuff. Photo sensors all over the aquarium read the ambient light level drop as a fish swims by and covers it with it's shadow. As the light level goes down, a corresponding inverse voltage is sent out to open an audio gate which allows one of several tracks of symphony sounds to come through an audio system mounted by the aquarium.

A digital multi-track recording of a symphony would be playing a series of different tunes at all times, but the individual instrument sounds would be silent until a fish swam by that track's corresponding photocell.

So a fish swims by the photocell mounted on the castle, the violin track fades in and out as it goes by. Another fish swims by the one mounted on the treasure chest, the flute section fades in and out, all within the structure of chord changes, crescendos, pastorale or fortissimo movements of the music tracks playing at all times. Every performance, even for the same song, would be different.

You could even do it for big municipal aquariums as well.

Add echo to everything just to tie it all together.