You had a mandarin, a maroon clown, a psudochromis, a majestic angel, a black & white clown, a boxfish, a hippo tang, a yellow tang and two percula clownfish all in a 75 gallon tank? If that is right that ends up being about 55 inches (or more) of fish in your tank!!!
First off, I think you have way to many fish in your tank (if you had all the fish I mentioned above at one time and not just having some replace others). When stocking you need to keep in mind the adult size of the fish and not the size when you purchase it. The fish per gallon rule isn’t concrete because some fish want a huge tank to swim in. A place to start with that is about 1” of fish per five gallons. So, by going with that you should have a 275-gallon tank for all those fish.
I would think your fish are all getting sick because they are overcrowded (or introduced one sick fish to the main system) and getting very stressed, which lowers their immune system and leaves them susceptible to things like marine ich. Now you have a lot of sick fish with what could be marine ich. You need to remove the sick fish and QT them till they are better (and all fish in the QT have shown no signs of anything for a few weeks) and like Alison said let your tank sit empty for a month so to make sure the ich runs it’s cycle and doesn’t have anymore fish to infect. With that many sick fish you are probably going to have to set up a few different QT tanks or tubs.
I don’t mean to flame, but I think you are have gotten into a big hole that might be quite difficult to get out of.
So, you need to reduce the number of fish you have and then QT the ones you want/can keep.
One QT thing you could do is lower the salinity to 1.012-1.010 for at least 3 weeks (really should have a good refractometer to be precise enough to do this).
Some people don’t like chemicals but another thing you could do is treat with citrated copper sulfate in the QT. In severe cases you could dip the fish in a solution of formalin (2 teaspoons of 37% formaldehyde per gallon of heavily aerated saltwater) for 10 to 15 minutes before placing in a QT tank.
I hope this helps some and hopefully others will help.
Also, I believe it is good to do regular small water changes (5% every other week, or 5% each week, or 10% every other week…or even 0.5 to 1% daily if you really want to). Large water changes of 25% to 30% tend to be more stressful on livestock and are not recommended for typical marine aquariums except in emergencies or major rescue efforts. When you get all your fish out you might want to do a large water change, though.
