Past and Future Pacu

The black Pacu is a fish that lives in the amazon river and grows up to 3 ft. tall. In the future, I suspect that these Pacu's would grow smaller if the water is warmer and there is less food, and also gain a lighter coloring to absorb less heat. I also gave it an internal and external mouth, used for extra ventilation and filtering food. The reason it would need extra ventilation and gills is because of warmer waters that may kill Pacu's that can't get rid of all of the heat. It would also likely have to move faster in warmer waters, which is why it get's longer and more aerodynamic.