I have 2 emerald green corys, one of them has a pinkish coloring on the skin, and the other has a yellowish tent, there is not much more discoloration in either of them. I have 3 julie corys that do not have these problems. I recently about a week ago changed the substrate from a black sand to a more natural color of sand due to adding plants and it being too dark. I initially thought it might of just been stress related, but its been a week and no change, worse or better. I have a 20 gallon tank, and im pretty sure im over stock, i have 4 platy fish that i am gonna get rid of because they poop A LOT and im pretty sure they are the reason all of my parameters steadily stay around .25 ppm. They only parameter that stays close to 0 is nitrate. All n all, i need help figuring out what is wrong with the 2 corys, the other fish do not have issues.