Before you use melafix I would suggest searching under tea tree oil which is its main ingredient. Its being touted as a safe cure but it has been shown to be toxic to a large number of animals. In addition, as it is a tree extract that can come from three different types of trees, there is significant differnences between batches, so one batch may not cause a problem but the next one may. It is also liver toxic, the damage from which can take significant periods of time to appear.
Shotgun treatments often do not work and if the animal recovers it is often in spite of the treatment not because of it. Many over the counter treatments and remedies are useless due to the resistance of the bacteria or other pathogenic agents (in this case for example, it is unknown if there is even an infection, or a wound, gill damage can produce these symptoms for example) and the only benefit that they have is that they allow the owner to feel as if they are doing something beneficial for the animal.
In general, even if a vet does not have experience with axolotls or amphibians, if they are willing to consult with other vets that do (such as those at a Zoo) then you are better off as the vet has access to diagnostic equipment as well as being able to prescribe effective antibiotics that are much safer to use on the axolotl. Contrary to popular belief there is a lot of good medical information out there but it has come about mainly in the last six years so it hasn't gotten out to most vets (and yes a lot are not interested in herps) but there is good info out there. If you have a vet who is willing to work with you, but you don't have the information to give them, you can purchase Amphibian Medicine and Captive Husbandry to take with you for the vet to review for dosages etc....
Ed