Cool tunes, I like the environmental art and personally I liked that it was pretty forgiving if you couldn't really hit the notes because... I can't really hit the notes. Nice use of the theme with fish scales and music scales at once.
I was thinking that it'd be cool if when I pressed the key with the fish in its spot, if it would then play an appropriate tone for the song. This could either be tones already within the song but extracted or perhaps better an appropriate harmony. I think that that would make it easier for me personally to find the rhythm. Maybe some kind of additional little visual cue would help too, in Hi-Fi Rush for example, they often have two concentric circles and when the big one shrinks down to the same size of the little one, you know its time to hit the button. So if you had something like that, and I could see when the fish was within the ring or whatever instead of obscured by the key prompt, I could hone in on that perfect timing better. Just various types of training wheels that would help me to find the virtual groove here.
Just some perspective from someone who is not a genre aficionado. All in all, this is a well put together package and I enjoyed jamming out with the fishies.