Okay, you're probably thinking to complicated.
First, check out which stone correspends to which song note (take a look at the shapes).
Then navigate all fish of each note color to the respective stone, until it starts to glow (all yellow fish to the top-left stone, for example).
The count for the fish is not always the same, in my case there have been more pink ones than yellow ones - but you have to get all of each color to the corresponding stone until it glows.
Have all 4 stones glow and you've solved it.