Did he looked very faded the days before he passed? That seems like what my radiata died too aswell, the angel picked on it, stressed it out and he wouldn't recover.
Sorry to hear about the loss, seems like radiatas are pretty senstive and just don't recover well from incidents. My had his fin nipped, and that's when he stopped eating and became pale. Same with the marine beta but the beta recovered the tissue, you can tell its white and not the same color as the rest of the body. I might try an antenata or just try raising a baby volitan if those aren't hardie either.