This reminds me a lot of an old popcap game I used to play as a kid, feeding frenzy. Might help with inspiration or ideas. Other than that I don't really like the movement, it feels very floaty and sort of frustrating to control. I think the game has a lot of potential though, especially if you added in some things like abilities, power-ups, or maybe special/unique fish with interesting mechanics and AI. Just my two cents though.