I don't have anything to suggest, but I wanted to wish you luck with your Pacu.
We had a Pacu a few years ago, whom I ended up naming Jaws, as she ate just about every other fish we put into the tank. Her natraul food that we fed her, however, was vegetable pellets, fresh vegetables, and occasionally meat (not fish). Anyway we had ours in a 75 gallon tank that we bought pretty much just for her when she outgrew her previoius tank. She was about five years old or so when we went on vacation and had my grandparents come over and take care of the fish for us. When we got back we found our Pacu doing the same thing, just laying on the bottom of the tank on her side. We cleaned the tank up, medicated it and did a water change and she recovered for a few days but then went back to laying on the bottom of the tank and breathing really hard. She was otherwise in good condition, no diseases known to us at all. The only thing we could think of was that she was very skittish and would often flip out when she was scared and ram herself into the sides of the tank. We tried everything we could think of to try to get the tank where it was supposed to be (all our values were good) and we just didn't know what was wrong. I wish I'd known these forums then. One night I went to bed (I was still a little kid when we had our Pacu) and my mother got tired of seeing her suffer so she placed her on top of the tank out of the water to kill her (my mother told me later what she did and said that her one eye had rotted away and some of her scales on the side that she was laying on). However, after leaving the Pacu out of water for 45 minutes, she was still alive. My mother placed her back in the tank, went to work (she works in a vets office) the next day, and called me at home and asked if she could bring home some euthanasia. I said yes, and hence our Pacu was put to sleep. I didn't want to do it, but with only one eye and rotting scales, not eating and struggling to breathe she was obviously suffering.
Like I said, good luck with your Pacu. I hope you cand find something to make it better.