Prasi isn't any good with bristle nose and scales fish. kurusi is for internal worms and wont do much for parasites. You need metro to clear that but if the others are eating fine and poo is showing worms you might have 2 seperate problems. I would start uping the water changes to 50% a day for a week or more get some epson salt in their too the big one might be clogged. If no luck and they don't improve Metro, I get it prescribe from a human doctor cheaper than vet and other suppliers. In Aus they come in 21 tablet 400mg per tablet. I would get the discus into a seperat QT tank if you can.
The red tail shark should not be kept with discus at all silvers are ok but not ideal and the neons may not like meds too much.