Melafix (and Bettafix, obviously) have been extensively tested on Bettas under all circumstances with no problems. I think the issue may be that Bettas are prone to bacterial infections, and the medication gets the blame if they unfortunately die.
Lilcavy, the chilling method of euthanasia is no longer considered humane, so you might want to look at alternatives.
Lilcavy, the chilling method of euthanasia is no longer considered humane, so you might want to look at alternatives.