I've been quickly experienced with marine ICH these last couple of months. I've lost 4 lil fishes. Last night being my last one. If only I'd have had a
QT tank.
I tried ICH Attack, which seemed to clear it up (from the fish only) but once I stopped it was right back. It did not kill the protozoa in the tank it's self. Only what was on the fish.
Hypo salinity can be dangerous. It has to be done slowly over time, which with ICH there may not be a lot of.
IMO the best thing for you to do is this. Remove ALL your fish from your DT, put them into a qt tank. Allow your DT to shallow for 6 to 8 weeks. The ICH will die off having nothing to feed on. There are many different meds out there that you can then treat your fish with.
Today my tank is fishless. I took my last one, a blue spot watchman goby to the
LFS where they gave me credit for him. That broke my heart. I had no where to keep him for the amount of time it would take for my DT to shallow. I am now looking for a 10g tank so I can set up a qt tank. Very hard lesson learned.