most directions for metronidazole is 50mg/l bath for 24hrs only, this seems to be a one time treatment although depending upon which condition it treats it can continue for 3-5 days (speak to vet before increase duration of medication).
I would normally recommend housing axolotls in 50% holtfreters + 0.1g/l magnesium sulphate as standard which helps prevent fungal/mold edema etc.. whilst improving water conditions, whilst the use of 100% holtfreters is as treatment for ammonia, fungal/mold, edema as well as parasitic organisms which are sensitive to salinity.
because general health can effect susceptibility to adverse conditions feeding a balanced but varied carnivore diet is required.
see here for holtfreters..
Axolotls - Requirements & Water Conditions in Captivity