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.