Fortunately, Ich reared its ugly head while my fish was in QT.
I removed 1 gallon of water from the QT and then dripped a gallon of fresh RODI until the tank was at level. Then measured my salinity. I repeated the process until my salinity was down to 1.010.
Additionally, I removed the fish and placed it in 1 gallon of RODI for 2 minutes daily for one week.
Then I gradually raised the salinity to 1.026 over a period of several days.
Once that was done, I monitored to see if there was a return. And I wait 90 days for everything.
Your situation is a bit different as it is your display tank. And corals do not do well in a hypo salinity environment.
So you would have to remove your fish and place them in a QT to do this. Also, I wouldn't place fish back into your display until the tank has been fishless for 90 days.
Here is a thread that talks about hypo salinity. It was written by
@Humblefish, who knows his stuff. Have a look.
https://www.reef2reef.com/threads/hyposalinity.247596/