30 fish spawn every page load all with random attributes.
The fish that survive the longest and creates the most eggs will have their successful genes passed on.
Fish that have mouths are carnivores and do not eat food and instead have to feed on other fish that are smaller than themselves.
Fish without mouths can only feed inside the food circles.
Every new generation of fish has small adjustment randomisations to allow fish to evolve and improve.
Every new refresh is a new ecosystem. Sometimes small fish that swim fast perform the best and sometimes big carnivorous fish do just as well.