Well I'm no pro but..what I would do is quarantine the rummy nose for 6 weeks. I'f they still look beautiful, then you can risk one discus. Add in one discus(hero discus) to the Tetra tank. If the discus survives after at least a few weeks, then you can add all the fish in together. If the hero fish dies, then he dies a martyr, saving all your other discus the same fate, but I'm just a newbie, I'm sure better answers will be posted in 3..2..1..