It seems to me that unless your discus are very small, you have too many fish for the aquarium. Also, it sounds like at least one of the fish is sick, in which case you should post your question (with pictures) in the "disease" part of the forum:
http://forum.simplydiscus.com/forumdisplay.php?f=34
As for the water change question, it's not really clear to me what is in the 2 drops you're adding to the water. Nitrites and Nitrates are the bad stuff you're trying to remove with the water change. Are you adding a "de-nitrifier?" if so, what brand etc. BTW, there is also another section in the forum that deals with water quality:
http://forum.simplydiscus.com/forumdisplay.php?f=35
Not trying to be mean, but you will likely get better help by posting questions in the correct sections.
Tim