As with any chytrid treatment, you will need to completely strip and dry out/disinfect the whole aquarium, as otherwise the axolotls will just be reinfected when they go back in. Have the axolotls been swabbed for chytrid? They are usually asymptomatic and other fungi could be implicated if the vet is just going from physical signs.
It is better to house an axolotl without fish, any way (they will damage the salamanders' gills and may get eaten in return - axies come from naturally fishless waters). Fish probably don't carry chytrid, but you will reinfect tanks just by moving water with fish on the animal and on nets etc, so they will need to be treated somehow. I don't know if it is safe to use Lamisil on fish, however, so that's something to ask the vet.
I can't stress enough the importance of disinfecting everything between lamisil treatments to avoid reinfection. This means the enclosure, the treatment container, equipment and hands in such a way that you don't track chytrid back to the animals after their bath. It would probably be a good idea to make up a water butt or similar of usable, uninfected tank water as you will need to do 100% water changes and tank sterilisation between treatments. Remember, your filter will also need to be sterilised.
