In order to rid your tank of ich you really need to setup a hospital tank move all of your fish to it, and treat them for ich. Then you need to leave your DT empty for at least six weeks, this will kill any remaining parasites in your tank and then move your fish from the hospital tank to your display only if they are healthy and ich free. Its a common misconception that ich is always present in your tank, and your fish cannot get ich if the parasite is not present in the first place.
Also how high are your nitrates? Nitrate levels of 20-40ppm in a fish only tank usually will have no effect on most fish and most are fine even with levels up to 100ppm although this is not optimal.