From the timeline it sounds like you have a long term ich problem, but the chromis likely brought in something much worse.
Does any fish have a red sore? - this would indicate uronema
Does any fish have peeling skin? - kinda like a sunburn skin flaking off - this would indicate brook
Does the fish skin look less shiny and more soft, dusty, easier to see when looking at an angle under light? - this would indicate velvet.
If none of the above apply, and the fish's fins look like they are eroding, this would indicate flukes - need a freshwater dip to confirm.
What is the fish behavior?