Keepin,
One does not know for sure, but you have done everything right - pristine water conditions, treated for parasites, aeration, etc. with no improvement. Just doing the same thing isn't going to change anything - otherwise your ray would be improving by now. The swelling that your ray shows indicates there is something internal going on. Metronidiazole is not harmful to rays - I used it on 2 month old pups when I had similar problems. I put some pups in a discus tank, they stopped eating, breathed heavily and started acting weird (were fine when I first got them). I did the same thing - water changes, Prazipro and all that happened was I got more stressed out watching my $3,000.00 slowly die. I had several conversations with the guy I bought the pups (captive bred) from. He is a breeder / importer / exporter from the US. As a last resort, he told me sometimes he has similar issues with imported rays where they wouldn't respond to the "normal" treatments that ray keepers do when their rays are in trouble and was told about bacterial infections. He was advised to use Metronidiazole. He used it and his rays rebounded. I used it and within a week, my pups were back to normal feeding again. I still have these rays today (ditched the discus though).
I think yours and DB scenario is similar.
I know I'm new to MFK and this forum, but I have been keeping rays since 2008.