Maybe it's just me, but the background is really noisy and distracted me from the game.
I loved the different sprites, it's definitely a cool idea to have fish in space! However, I think the idea with inflating the fish and deflating it wasn't that well executed. It seemed to me that staying a small fish was infinitely better than being the bigger fish, just because I didn't really feel the need to destroy the asteroids.
Talking about the "bigger fish mode": I think it would've been cool if you could see which objects can be destroyed and which couldn't. I know, it was explained in the tutorial, and maybe I'm just dumb, but I had already forgotten all the objects that I could destroy. Maybe destroyable objects could have a indicator, maybe an outline, or a specially broken appearance.
All in all, great idea, but a few imperfections here and there.