Your mistake was not in the water quality or parameters, but the lack of quaranteening the new discus. This is very important for all new fish, but for some reason, discus need proper QT time than others. The new fish had a different bacteria on them that the other discus in the tank did not...this caused them to get sick and die. And the new ones are not doing well because they have not been exposed to the current bacteria(s) in the tank...so they got sick as well. If you QT them in a separate tank for several weeks and allow them to acclimate their bodies to the water and bacteria found in the water before putting them into the tank, all discus would have done MUCH better.
Also, a point was brought up earlier in this thread regarding the health of the discus bought....this is also very important. DO NOT buy your discus from your LFS. This is just asking for trouble. buy them from a local breeder or if there are none to be found, then buy them from a reputable discus breeder/importer who can ship them to you. The reason why I say avoid LFS discus is because theyare most likely discus imported from brokers in Singapore or Thailand for very cheap....couple bucks each. Discus bought from brokers run extremely high risks of being sick due to multiple sources without proper QT time. They go around to several farms, buy fish in bulk and bring them altogether in one facility for a short while before they are exported out. And most of the time, each farm the broker buys from only specializes in 2-3 color types at most. So if you are importing a whole array of colors from a broker (which most wholesalers do), you are bringing in fish which come from multiple sources and have not been properly QT before export. And because of this, they usually begin to break down in the wholesalers tanks shortly after being imported and may die. This is the primary reason why discus are hard to find in a LFS. They dont want to spend the little extra money to buy discus direct from breeders. Wholesalers only care about who has the cheapest fish...and with discus, cheap is most certainly not good. Other times, the fish do fairly well once imported, but dont do so well once they are shipped to a LFS. This is because they are kept in less than ideal conditions (low temp, dirty tank, no w/c, etc...) and may even be mixed in with other sources of discus. All of this stress destroys their immune system. This all leads to a time bomb effect. When you buy your discus and bring them home, they will start out doing good, but shortly after start to degrade in health. And lets not forget to add in hormones to the mix...which compounds things even more. Hormones are used to color the discus at a small size...so a 3" discus which isnt supposed to show its true colors until atleast 5" will show full adult colors once treated with hormones like Methyl Testosterone. Hormones will severely damage and scar internal organs such as the liver, kidney's and various others. Then organ failure will set in which leads to the fish dying.
So, to sum it all up....properly QT your new discus in a separate tank for several weeks....4 weeks minimum is recommended. Buy discus from a local breeder. If you cant find one local to you, then buy them from a reputable breeder/importer who can ship them to you. Cheap discus are never good discus....you get what you pay for when you are dealing with discus.
-Ryan
Orlando Discus