Fish is still hanging in there
Not eating, still breathing heavy
Since the fish WAS eating well in your tank, you can eliminate issues with transport or acclimation. Since the other fish are fine, you can rule out water quality issues. The length of time that it has lived like this rules out Amyloodinium (velvet) and mostly rules out bacterial gill disease. That narrows things down quite a bit: residual toxicity from being collected with cyanide, some viral issue, coccidia or myxosporidians. None of those can be treated. The only treatable disease I can think of is gill flukes, thus my earlier recommendation to try a FW dip and/or prazipro. Flukes can be specific to certain groups of fish, and they are slow to kill, so that all matches up. However, going this long without food makes any treatment more risky. Also, what can happen with heavy fluke infections is when the fish is treated, all of the flukes fall off at once, leaving hundreds of tiny holes in the fish's gills and skin and then the fish bleeds out.
Sorry - but I just don't see a good resolution at this point.....
Jay