Any chance the tank is planted and your running CO2? Your staple food is not expired and has not been exposed to the possibility of chemical contamination of any sort?
Aside from the plecos (we can come back to this in minute), there is nothing that would cause even mild concern here. Disorientation/dizziness followed by demise can sometimes be linked to a vitamin deficiency or to more obvious problems such as ammonia, nitrite or chlorine poisoning. Oxygen deprivation can cause these symptoms as well. Electrical current can cause the same symptoms. None of these appear to apply here and your tank is well established with solid parameters and is well maintained. Your feeding a quality food source as well. You have not changed or added anything.
So, let's return to the plecos. They were the first to die, correct? Aside from the fighting, did they appear healthy before you added them? Absolutely no even slightly odd symptoms or discolorations? Did they die immediately (within 24hrs) or after days? Any symptoms at/upon death? I am trying to figure out if they were the source of whatever may be affecting your tank or if its just coincidence that you added them and then something went south in the tank at the same time that is not disease related.
There are some uncommon viral and bacterial diseases that can affect the brain and CNS causing odd symptoms and demise. Viral diseases are not treatable and generally infections in the brain and CNS are difficult to treat. But its odd that the plecos showed no symptoms prior to adding them to the tank. As everyone is eating well, an option is to start feeding them an antibiotic food (low impact on bacteria) or dose the tank with antibiotics in random hope that its something bacterial that is treatable.
I am honestly baffled right now as to what may be happening or why. And with other healthy tanks that do not have this issue, your water source itself as a possible cause can be likely ruled out but I am afraid I don't have a specific answer as to why this is happening. So, I am going to think some more on this. Hopefully, someone else may have some ideas or something that I am missing here!
In the meantime, do you by chance have methylene blue? I honestly don't know if it will do anything at all here because I do not know what the cause is but it will not hurt to try putting a fish that is vaguely starting to show symptoms in a methylene blue bath. Just a random thought as it does help increase blood oxygen levels.
Let us know if you can think of anything else (even something minor) that might help figure out this mystery.