WASD, eat smaller fish, avoid bigger fish - including sharks because they too are big fishes. When you eat a smaller fish then you gain +2 size so you can eat more fish. When you touch a bigger fish then you get eaten by the bigger fish and you lose.
I can't really make the game any better from the smaller and bigger fish clones point of view because of the lack of a few more blocks about sprites or specific clones, like: "When green flag clicked, if - touching 'fish'/----'FISH CLONE'----, and (size < ----'FISH/FISH CLONE'----) then..." "When I recieve 'messagenomnom', delete ----'FISH CLONE' if it is/was ---- <touching 'YOU'>...