If they are not showing signs of Ich, I am going to guess stress and/or poor water parameters. Angels and Tangs seem to not handle stress or disease/parasites as well as other fish, so im not surprised that other fish are not as bad off.
I do not want to come across as condescending here, but a couple of thoughts/suggestions that i feel will probably help in the long run...
(1) IMO, in saltwater, you kinda have to have a test kit and know your parameters. I normally only check Nitrates, because if the tank was cycled properly, your ammonia and nitrites should stay zero on their own. I test weekly and do partial water changes about every 2 to 3 weeks in my FOWLR tanks. My nitrates tend to stay low doing that, so I stay with that routine. I know many do weekly PWCs and if you need to to keep your trates down, then that's what you do. Its almost impossible to just hope for the best without a kit though. Doing a water change is good, but you need to know what your tank is at afterwards...a day later...a week later. Pristine water is what your fish need now.
(2). I am a huge proponent for fish being in proper sized tanks. Stress kills. Almost every time you read a thread on here (or other sites), fish with issues are in tanks that are smaller than suggested. Hundreds of examples can't make that coincidence. I go with liveaquaria.com for reference, as I feel they tend to go bigger for their minimum sizes and I tend to see that bigger works better...smaller simply causes stress issues. People will start the "tang police" nonsense, but it holds true for any fish, not just Tangs. You have a Sailfin that should be in a six foot tank in a four foot tank. That causes stress, no matter how the fish seems. I also believe the size of the fish does not matter, so "its tiny" is not a factor (IMO).
Outside the Tang (which I would rehome if it makes it), you have damsels which are notorious for being bullies. Clowns can be territorial too. Bullies cause stress on tankmates. Hawks can be tricky sometimes...not a tank size issue as much as personality. I had a Flame in a 300g that chased any fish that got near his perch and he would chase them all the way to the other side of the tank.
If white spots appear, its another story, but for now, I would get a kit, start testing water regularly and try to shoot for zeros across the board. Pristine water should help the pop eye. I would get rid of the damsels to alleviate one of the stressors. I would rehome the Sailfin, especially if it had no signs of illness. Its simply not going to thrive in a 75g in the long run.
Sorry for being so wordy...again, I'm trying to help, not knock you.
