I have a fish that had an internal parasite. He went from being the biggest to being the smallest. He is 2" while the rest of the hatch is 4"

As spongebob said, the fish will have a different personality when not overstocked. Since I upgraded to larger aquarium and got the herd of platies out of the tank, all my tetras joined their schools and have class all day now. Before then, it was a free-for-all in the tank, everyone would swim together and couldn't tell heads or tails of the different fish. The plecos only got as big as the area they were in, stayed 4 and 6 inches in the 10 gallon tank, grew to 10 and 12 inches in the 30 gallon tank, and one is already at almost 14 inches now in the larger tank.