I had this problem as well. I started with 2 mono's and 4 orino's. I'm now down to 1 orino. All of the fish came from high ph water - around 7.5 and thats what they went into in my tank. This is high, but the fish were definitely acclimated to it, considering the monos came from an LFS and the orinos came from Rich in SoCal.
The problem happened one at a time. I'd wake up and feed them in the morning and one wouldn't eat. I'd get home from work and he'd be breathing hard. The next morning, he'd be dead.
I asked the same question on here as you and got various answers involving water temp, parameters, etc. I can assure you, my parameters were spot on, the tank received 50% water changes weekly, the temp was around 85/f and I had two air stones in the tank. I even took one fish out and put him in a hospital tank.
I've yet to figure out what killed my fish. When only one was left, I just tossed it in another tank and stopped caring. The temp in that tank is around 80. I stopped offering him special care, however, I still do try to feed him twice a day, but other than, I just keep up with my water changes. He's now been in there around a month. He shows no signs of ick or any other disease. However, he did jump out of the tank once when I was feeding him. He literally jumped right into my lap.
The best answer anyone gave me was the guy at the LFS. He said that bacteria will grow much faster in warmer water. So even though my water was fine when tested, the bacteria wouldn't show up on the test and was probably still killing my fish.
So with that, I'd say my advice would be to put them in their own tank or a quarantine tank that is smaller. Drop the temp to around 80-82 and see what happens. If you start to see signs of ick, at least this way it'll be easy to raise the temp back up and medicate for that.