If thats the case I would have to agree with water changes. it can help oust a parasite, BUT you may not be able to get them all.
if worse comes to worse, I know its a pain to do, but you can move your fish into a qt and treat them for parasites there. But theres stillthe chance that the parasite could remain in your tank and reinfect the fish again.
The red gills is also a sign of ammonia poisoning as well just so you know. I woudl watch those fish like a hawk.
And not to be off topic, but a yellow tang for sure does not belong in that small of a tank, not sure why your LFS
would recommend one or sell it to you unless he didnt know what size tank it was going in. Most tangs require at least a 125g 6 foot long tank. Some like the Kole tangs can be ok in a 75g but theres only a select few that should be in that size.
The tang may have come in with the parasite or the stress of the new home and the tank size could have made the parasite present itself.
Again, so sorry about your losses. It does happen though and it sucks.