Each fish in "El-Fish" has a unique Roe, similar to the genome. This allows the user to catch fish and use selective breeding and mutation to create fish to their own tastes for placing them in virtual aquariums. Around 800 possible genetic attributes (fin shape, body color, size, etc.) are available, which can be selectively shaped into virtually infinite numbers of unique fish. Once fish are created, "El-Fish" will algorithmically generate up to 256 animation frames so that the fish will appear to swim smoothly around the tank.