Axolotl Taking Chunks out of his tail

[ArchieThaAxolotl]

Hi, I'm new to this site, this is my first time forumming. Anyways, My axolotl (Archie) has been taking chunks out of his tail and I don't know why. you can see it in the png provided. He is a healthy weight and is in a 20g tank. Water parameters are 15 Nitrate, 0 Nitrite, 0 ammonia, 7.6 PH. As you can see, he eats well and is in a nice enclosure. (The rocks don't have sharp edges) he is almost 2 years old. plss Help

 

[wolfen]

looks like he might be getting it caught, possibly between the rocks, doesn't seem to be affecting him much and it will grow back.