Ok, have read the previous post. I have just divided my large tank into two to accomodate some of the larvae. Jeffrey has the large side and the 4 youngsters have the smaller section. This has been done to as the younger axolotl are getting too big for their tank. When I seperated the tank, the remaining sand was moved all into the adult side, as I feel this will be easier for me to clean the youngsters side of uneaten food. Now Jeffrey was able to see/ scent movement from the other side of the barrier and this caused him some upset. As my husband and I stood watching he snapped at a piece of floating plant bouncing along the bottom of the tank ( I think he thought it was dinner!) , in the proccess of doing this he literally took a huge mouthfull of the sand...he then proceeded to 'puff' it out through his mouth and possibly even his gills. Now I have witnessed him do this I have no doubt that sand/gravel/ stones will be inedvertantly be ingested along with what ever food is presented, especially if it has contact with the substrate..i.e fallen earthworm, blood worm etc.
Up until this point I too would have said that my axolotl , who is mainly fed by hand would not have any interest what so ever in consuming his substrate, now I know otherwise. In the photo of your little axolotl I can see a number of stones in just that one section that could be ingested in error, much as I like to gaze in wonder at my little beasties I can't watch them 24/7...who know's what they get up to when you are not around ( mine certainly managed to produce a hoard of eggs!!!).
At the end of the day Stephen I can only ever offer you advice on what I feel is correct and in this instance I would feed your little guy daily if you feel he is under weight and to remove the gravel as this could contribute in endangering your little guy's life.