Demyx is a chronic biter as well as badly bred, prone to every disease under the sun and stunted by bad conditions in his tiddlerhood, and oh boy do I know the feeling of "I give up".
So basically, I put him in a very small (1.5 g) barebottom tank so he could cope with trying to swim with his stupidly massive ever-raggedy fins, dropped a handful of peacock moss and a fake sword plant in there for him, keep the water pristine but otherwise, basically left him alone.
So that seems to be working, as he is now growing out his ludicrous fins after 6 months of total panic & drama - seems he's a rosetail and not a HM, which explains a lot.
Giving him all care and stacks of moss (with a little IAL as well) but basically quitting worrying myself to death over whether he's going to live or die has meant he's still his happy little weedy self, but the stress level on both of us has dropped significantly.
Way less fin biting, and therefore way less infections. I think it's worth a try.