****** is contageous, flukes only require one host to reproduce, and thus, are much less contageous.
you'll need to remove the fish to treat it.
your water quality is AFU...your pH is dangerously low, however, the low pH is likely keeping the high NH3 from killing your fish. as mentioned, you're going to need a few huge WC's and will need to clean your sump out. you need to be REALLY careful with adding a NH3 neutralizer, as most will crash your pH worse than it is...i'd go for WC's, personally.
it may be best to pull the fish while you get your tank under control, because once the pH rises, the NH3 will burn your fishes' gills. it's also possible that the poor water quality is the source of your foxface's eye trouble.
if i were you, i'd get the tank under control first and foremost.