It doesn't seem like your following the Nitrogen Cycle from what I can tell.
Using well water doesn't mean your exempt from harmful substances, well water can still contain contaminants which you generally can only be aware of my contacting the authority that manages your water supply and asking for test results of what substances your well water contains.
Also the Nitrogen Cycle is one of the most important factors of keeping your Axolotl healthy, its imperative that you test your water as often as possible, usually every week or so. The water will develop harmful toxins as your Axolotl excretes waste. Your Axolotl's waste and any decomposed food in the water will create Ammonia. Ammonia is a deadly toxin that if allowed to reach high levels can cause stress, fungus, disease, and potentially death. You need to 'Cycle' your tank.
The cycling process can take from 3-6 weeks.
For your tank to be cycled, a beneficial bacteria needs to grow and colonize your tank to convert the toxic Ammonia into Nitrites then these Nitrites into Nitrates. Ammonia is lethal to Axolotls and is produced from your Axolotls waste and decomposing food. Nitrates are slightly less toxic and created from the Nitrites being converted by the beneficial bacteria.
During cycling the beneficial bacteria will colonize your tanks surfaces such as substrate, ornaments, filter, and first begin to feed on the Ammonia turning it into Nitrites, then bacteria will break down the Nitrites into Nitrates. The bacteria is known as a 'biofilter', as it filters the ammonia and nitrites into less harmful nitrates. Then you can lower the Nitrates by performing weekly water changes.
Caudata Culture Articles - Cycling
Caudata Culture Articles - Water Quality
Your juvenile Axolotl does seem to be suffering from poor water quality, the fimbriae on the gills look quite shriveled and small, in comparison to an image like this:
http://www.caudata.org/forum/members/axolotlchris/albums/axolotl/37846-back-legs/
Can you test your Ammonia, Nitrites, NitrAtes, and PH, then post the exact results?