Sorry we seem to have missed this one ...
Initial observation is that 2 days qt isn't qt - ideally six weeks, minimum ime is 4 weeks.
The white pimples you see from time to time are indicative of the water quality not being quite good enough. Upping the frequency of water changes and wipe downs will help resolve that, even one more 40% change in the week may be all that is needed.
You can drop the temp a little too, 28C is fine for discus and will save a little on electric bills.
As for the one gill, I'd see how the increased water changes go for a couple of weeks. If it gets worse, or other fish start to show the same symptoms then let us know.