Please do not keep fish in the same tank with axolotls.
Most fish will chew on an Axolotl's gills and certain fish also have defence measures such as spines that will damage the axolotl when the axolotl trys to eat it.
The only fish you should keep with axolotls are feeder fish. But as your axolotl has damaged gills, no fish should be kept with him - not even feeder fish as they will attack the injury.
While the axolotl is sick, keep him isolated to prevent disease spreading to the other axolotl.
I would definately fridge him. Prepare a tub of fresh dechlorinated water, place the axolotl in the tub and place in the fridge. Keep bottles of dechlorinated water in the fridge as well - these will be used for the 100% water changes which must be done daily.
Could you please post a photo of your axolotl?