I just saw that you did post your water parameters over on the other thread. Mods should combine these two threads in one place. Here's what I posted over on that one:
I just read through your other posts since joining. Looks like you just finished cycling your tank around the 18th of september? So your fish stock are recent purchases? If so, your stock could have been sick when you purchased them. I hate to recommend meds when the illness hasn't really been diagnosed yet, but if they are new and they may have already been sick when you purchased them, I'd treat with a combination of maracyn and maracyn 2 which the combination will knock out a lot of bacterias. But do a partial water change first. It is not uncommon to purchase cories from the pet stores these days and have them be in very poor condition only to die shortly after you purchase them.