Pretty cool idea of having an animal grow and eat other animals so you can become the king of the ocean. I got up to a D+ fish, but couldn't figure out exactly what to do after; I tried reincarnating but it looked like the 2 reincarnation options were the same fish, which felt strange since I got up to D+ before, but these other options reincarnated me to E. I also thought the plankton that you eat as an E fish could've spawned more frequently, as I spent a lot of time just dodging fish trying to find plankton to eat.
Really solid idea, with good graphics too; could use some music as well, but overall this was a solid entry, great job!