Poor little guy, it's tough to know what to do! I've never kept Black Widow Tetra myself, but I know their rep for being nippy if not in a large enough group, and that they establish a pecking order in their groups, I think? So perhaps because he'd been hurt, weakened and then been gone from the tank for a while and then introduced, it makes sense that the other fish now see him as a weaker outsider and he might struggle to be accepted into the group again, especially if he still has some weakness like you said.
Since I don't have much experience with tetra, going to tag
@Slaphppy7 and ask what he thinks? Whether it might be worth giving him more time to recover, then trying out removing the rest of the widow tetra from the main tank, re-arranging things and establishing lots of hiding places, then re-introducing him to the main tank with some of the more chill/lower ranked tetra first, to mix up the territory/pecking order, the way people sometimes do with cichlids that are bullying?
If a second later attempt at reintroduction, or an attempt to try him in the tank with the pearl gourami doesn't work out, then he may have some permanent damage that means he'll need to remain alone. I'm really not sure, it would have to be a judgement call you make. Wishing you and fishy luck though! Would definitely like to know how things turn out, if you're happy to update us!
So relieved I didn't euthanase him when things looked at their worst. Thanks for your help.
