What kind of fish mixed with a discus?
You could put most peaceful fish in a large tank for discus just as long they can tolerate the water chemistry. The water chemistry for discus is soft between 3 and 15 DH. The PH should be between 5 and 6.5. But I have seen it written that discus can be infected by germs from other fish and could kill them. Here is a guide for discus tank mates.