Genetic Variation There are more red fishes than blue fishes.
Overproduction of Offspring/ Struggle The red fishes are more easier to spot, so the its more easier fro them to get eaten by the sharks.
Differential Survival and Reproduction Since the red fishes are easier to spot, their amount is decreasing by a lot; the blue fishes are more harder to see, because they sort of camouflage into the water, leading to their amount to increase and have more blue fishes.
Change in Gene Pool The change in the the gene pool, is that at first there was more red fishes than blue, and now there is more blue then red. This happen, because the red fishes were easier to spot and they had a disadvantage in the environment they lived in. The blue fishes had an advantage, because they aren't easily seen in the night or night because they blend into the water.