I think ammonia burn should and always (?) does produce some redness.
It does not look the case. Just a lot of slime coat coming off, which means they are very much stressed. The stress factor is up to you to figure out. Seems like water (although could be a disease). Tap water or the incoming water must always be considered a suspect. Double check the test results with another kit or at an LFS, friend, etc.
So much stress from ammonia I'd think would indicate a high level of at least 3-4 ppm. This means that fish would lose their appetite long ago and all of them. And all of them would be very, very stressed and breathing heavy.
Ammonia cannot be not registered by a liquid test kit. If it's out of the high range, the test will be more colored than the highest reference point. Almost black.
Nitrates must always read non zero IME and from reading. Even in heavily planted tanks NO3 is around 5-10 ppm IIRC.
IDK the details of your tap water treatment. Carbon has a limit of adsorption and must be reactivated timely or it is known to sometimes release whatever chemical compounds it had adsorbed in a flooding manner. But that would affect all fish.
In summary, if all other fish are good, look good, eat as usual, then it is not water. It may be a disease specific to your paroons. If other fish are also not themselves, it is most likely the water.
Are you well understocked and well overfiltered? We sometimes forget about these basics.
