you do the salt baths outside the fridge.
if your axie is poorly and being fridged you make sure there is a bottle of dechlorinated water in there also, this is what you will use for salt baths and water changes [daily]
the salt baths should be 10 mins 2-3 times a day.
2 teaspoons of salt per litre of water.you dissolve salt in a container of fridged water , take axie from tub and pop him in bath, whilst he is in there it is a good time to do water change on tub. then you can pop him back in tub and fridge.
if he isnt in the fridge you just use dechlorinated water approx same temp as tank.