Well, creating ideal temps, water quality, and providing balanced nutrition are generally the best way to promote the best traits in a healthy axolotl. That said, if metamorphosis has actually begun due to stress it is not possible to reverse it. Also, an axolotls gills only really are as long and full as they need to respire in their environment.
Too much gill can actually make them sensitive to very oxygenated water. Alot of aeration can promote gill shrinkage, but it also makes oxygen more available making thriving easier.
Air stones and bubble walls are strong aerators,
So personally I would say it's healthiest to let your axolotl grow it's gills as long as it needs rather than try various environment-modifying tactics to promote an aesthetic appearance of gills. Gill-length is no direct measure of vitality.
The axolotls gills probably were damaged by toxic water. With improved conditions, and absence of detrimental infections, the gills will grow back to a length that best serves the animals oxygen requirements. This tends to be less with older animals, or those in cooler, less-stressful environments.
We were away for 3 weeks and left our axis with a friend. One of the tanks if fine but the large one with 4 axis all the their gills have shrunk. I have changed the water because it was horrible i think he was feeding them way to much. No fungus or anything just no feathery parts on their gills its all gone on all four of them. How can i help them get their gills back???