Ah, mick, I'm sorry. Euthanasia was needed in this case.
Formaldehyde can be a very dangerous thing with fish. Did you treat them at full dose or half? I agree with Diana, you may be able to save some if you can get some antibiotics, but when it hits this fast there's little time.
I'd see if you can hit up Emma for a sympathetic veterinarian referral. In the meantime, yes, you can topically apply either melafix or even honey as was suggested, but in an already stressed out fish, that may do as much harm as the treatments Diana mentioned. And in the case of both melafix and honey, they are not true antibiotics and are not likely to be potent enough to kill off the flex.
I'd remove all botia species to a qtank, watch the other tank closely, try your damnedest to lay your hands on some antibiotics (since flex is contagious) and plan to treat both the qtank asap and the other fish that were in with the botia as soon as you can. You don't want this in your tank.
If you don't kill it off, it will eventually rear it's ugly head again when a sick or stressed fish comes down with it.
So sorry mick.
