Thanks for your detailed feedback.
The idea of highlighting the fish did come up during development but we couldn't add it in time. This is no excuse but rather something to think about going into our next project, so I appreciate it! It also synergizes poorly with the jump in difficulty that's been noted to us, and not a fun kind of difficulty but a frustrating one. Polishing the levels will help this too.
Personally, I can be responsible for the graphics and strive to improve so there's something to write home about next time!