see popeye is first caused from bad water ie high amonia/nitrate! but i dont think that is your problem as tetras are weak they canot handle high levels so they would of been droping off first which would of warned you ?
bactiria behind the eye is formed after the eye is poped out which is the major pronlem so if your treating it for that thats great. (sorry not sure wat the twins are)
sorry for the loss of your first plec did it die from the same problem?
also try not to stress your fish out by try to see it every day or 2 or 3 times to see if it is inproving that will only just stress it out too much
p.s find what caused it, was it a fight! scratch? or was it bad water? need to remove or fix that problem as your already doing WCs (as said before) so thats great!
all the best!