My discus were pretty happy. That was until I bought 2 more, a Tangerin Dream and a Blue Turquiose. Being naive, I put them into my 90G that very evening. In a couple of days, one discus got sick. Of course, my hospital tank I didnt bother to keep running so I had to quickly get it started again plus cycled. Well, that took a week so by the time I finally was able to put my discus into the hospital tank, one was pretty much a goner and two more were infected. My discus are healthy now but I lost two more before they were. At least the new ones survived which costed me $100 for both of them. I still am fighting a disease in my 90G. My dwarf neon rainbows are sick and I have lost 4 rasboras and 1 cardinal. I hope that it will to over soon.
Now I know better. Use the hospital tank no matter who you get fish from. The lfs I bought the discus from is the best and I have bought many fish from them without any problems. I know their fish are the best however no one can be garenteed that the fish are %100 disease free.
Another cause might of been my pH fluctuating because of my C02 injection and surface movement from my bio wheel. Even though the movement was very slight, my pH would keep rising to 7.6. I would then increase C02, it would come down however I was approaching the harmful level of C02 of around 25-30ppm so I could not increase C02. I finally removed my bio wheel and now there is no surface movement. C02 injection is a lot less and pH is steady at 7.0. That should help with the disease.