Mystery Injury Help?

[ClockworkCan]

So I have had my darling axolotl, Skeletor, for a couple years now but today I found this weird injury/hole in his dorsal fin area. The actual injury isn't that concerning because I know it will heal but I'm more concerned for what caused it. I haven't manhandled him or anything and I haven't seen him spooked or thrash around recently so I haven't a clue. I worry about some sort of "invisible" harm like a chemical issue or disease I don't know of. He is otherwise healthy and I included a picture his head to help the pet detectives out there.

If anyone could offer some insight that would be lovely. Thanks!
Pictures attached below

 

[Murk]

You haven't seen him spook or trash, but he could very well have done this when you weren't looking
Sometimes, they just get spooked for no reason other than that they're silly creatures. It doesn't look like there is too much sharp or ragged edges in the tank, but either of those ornaments could very well have chipped of a piece of axolotl.

He seems otherwise healthy and happy, so I wouldn't be worried yet.
I'm not a vet, though. Do keep an eye on it.

 

[ClockworkCan]

Okie doke Thanks. My mom instincts kinda kick in when it comes to him.

 

[Murk]

That's only good, right?
Ironically, I would always advise to follow your gut feeling over the advice of random amateurs on the internet!