Well, I don't know what further I can do for you. I'm sorry, but that's the truth. You need to catch the diamond goby because the ich will try and attach to him because they will not have any other host. I really cannot tell you why you might be having issues because like reno said, you do not really know any of your water levels. Alk is extremely important in a quarantine tank, and you really need to have at least a pH, kH, and nitrate test kit when you do hypo on a fish, because these levels tend to jump out of control. And a refractometer is a must-have while you do hypo because that is really the only way to ensure that the hypo is really hypo (1.009). I don't mean to be the bearer of bad news, but that is the truth. These are the possible consequences when you do not use a quarantine tank for all new fish introductions.
Good luck with everything, and I will be here to help you if you have any further questions.