Frequently used as a preventive antifungal for developing fish eggs, methylene blue can also be used as a treatment for fungus (saprolegniosis) in sensitive aquatic species such as axolotls. Methylene blue also aids in the recovery from nitrite poisoning, cyanide poisoning and methemoglobinemia by returning red blood cells to their regular oxygen affinity.
While methylene blue has some antibacterial and antiparasitic properties, there are better alternatives in most cases. However, it is effective against Mycobacterium ulcerans, which causes buruli ulcer in humans. While no cases have been reported in Canada so far, fish and amphibians have been suggested as potential hosts for the pathogen. For this reason, I like to do a preventive methylene blue treatment when quarantining new arrivals who may have been in contact with foreign species (e.g. animals coming from a pet shop.) If you get your pets directly from me, you may skip this step.
When treating with methylene blue, I recommend keeping your axolotl in a separate plastic tub. This will allow you to fridge your axolotl if needed, while keeping your filter bacteria and plants safe (methylene blue is toxic to both). If you really must treat the whole aquarium, remember to remove any chemical filtration during the treatment. After treatment, use chemical media and water changes to remove any leftover medication, then add a bacterial supplement to restart your cycle.