I started my tomatoes from seed and have repotted once. They are about 10 inches tall. However their leaves have started to turn purple on the underside and curl the die. I read it may be from overwatering so I let them dry out and that didn't seem to help. They are being grown inside under grow lights. There is currently 3 seedlings per 4x4 pot. Should I thin them out and put in bigger pot? Idk what else to do. There doesnt seem to be any disease or bugs. I fertilized them hoping that might help.