Hard to guess here. If a cc. More would be sick. Mucus/slime showing on their body and all grouped up on the top of the tank. 25g tank with 9 discus is a problem. Even though you’re doing the water changes now what have you been doing before the problem? How often and how much. With that many discus I think your tests may be off. I don’t see filtration. Is there any? You need a bigger tank. 75-125g for that many that are that big. Unless the picture is making them look big. They look like 5” discus.