Piranhas dying

Question
We bought these red bellied pirahna's two years ago as babies and they have always been very healthy until one day my husband decided to cover a portion of the tank with thick wall paper to stop the lorikeet from pooping on the tank. My husband fed them their usual chicken breast that morning about five days ago and left for work. He said they all ate fine and appeared healthy. When I checked the tank at around 11am that same morning I noticed they were not doing well and they had gotten cloudy patches on their eyes as well as were not swimming very fast at all nor reacting like they usually do to light or sudden movement. Their eyes got worse very quickly to the point you could barely se their pupil they were just glazed. I put a little melafix in the water with prima fix and a tad bit of salt (epsom salt) before my husband got home and did the 25% water change. One of the pirahna's was really bad off and was swimming upside down by mid afternoon. He was having what I would describe as little seizures from time to time where he would just start trying to get out of the tank almost. I decided to change the filters and I lifted up the wallpaper which had been there for almost three weeks and saw black mold growing on the fish tank. I removed the wall paper but by the time my husband got home and was able to do the water change the one had died. The other five were still lethargic with two of them starting to get the same issues as the one that had died. Really fast breathing and swimming upside down having fits occasionally darting all over the tank for a few seconds before returning to barely hanging on. They also just kept trying to gasp for air at the top of the tank during the water change. We continued to do water changes slowly for the next two days and they are fortunately for us still alive with the other three seeming to be recovering well. However they are bad off. REALLY bad off just struggling and laying on the bottom of the tank basically slowly deteriorating from the eyes to the flesh just barely breathing but alive nonetheless. My husband wants to put them out of their misery but I am trying to wish for recovery. They are really bad off now though after the last four days of laying on the bottom or floating upside down. Their skin is literally just rotting it appears and their eyes are sunken in but they are breathing. I don't know what to do. The other three are really doing well and are eating. I am afraid if I don't do something then the other fish will get sick again since the two fish are obviously rotting in the same tank. Please help.

Answer
Christina,
you are in a bad situation as you are aware.
I wish I could offer better options, but , if I were you
I would try  a seperate tank for the one that are in trouble.
At this point you are into secondary infections beyond the obvious trouble.
If you can quarrentine the sick ones, add some meds to combat the tissue rot.
Some stress coat, temp 80 plus, acidic pH, this will keep the infections down as well.
All you can do is hope they can recover at this point however it does appear doubtful at this time.
I would continue to monitor the others and the levels in the tank as well, ph, temp, ammonia, nitrates.
I wish I could offer you something more but at this point the options are limited.
good luck and you are to be commended for at least doing the best you can for the fish.