Help!

Alright guys, so I have a 60 gallon tank and recently I had my tang get Marine Ich which I treated him for and he is doing great now but within the last 2 days I have woken up to find 3 dead fish and 2 dead turbo snails. I understand that the snails have a hard time with certain medications for Ich but I can't figure out why the other fish have died. 1 clown, 1 damsel, and 1 orchid dottyback. I have done numerous water changes and all my parameters are good and I have actually had other stores/people test the water just to make sure I'm not missing something. So far no one can help. It almost looks like the fish have ich still (I light white powder dusting over their bodies) but if it is Ich why did the solution I used work for the tang but not these other fish. Any positive feedback and help would be great.

Thanks,

Matt