Photo: My axolotl a new black dot on his side/under belly that looks strange.

I just noticed it there last night and was wondering if anyone could give me some information as to what it is and why it appeared there.

Is the spot raised or smooth against the skin?

Uhm, I don't know if this could be it, and please bear in mind I'm far, far from an expert, but I did some digging around and found two possible causes:

1) Axolotls can get (sometimes benign) lesions, tumors, bumps, or just discolorations, that are just basically collections of oddly colored skin cells. Someone else had an axolotl with a similar spot, it appears on this thread: http://www.caudata.org/forum/f46-be...my-axolotls-head-its-growing-very-slowly.html

2) I have no idea if this can affect axolotls, but there is a parasite fish can get that leaves black spots, called Black Spot (diplopstomiasis). It's caused by a parasitic flatworm. Here's some information on it occurring in fish: Aquarium FD - Black Spot (diplopstomiasis) - Disease Identification, Diagnosis & Treatment

Again, I am far from an expert, and I don't even know if the black spot disease can affect axolotls, but given how I didn't see many replies here I figured I'd throw in my two cents. Your best bet is to contact a vet, if you can, of course. (Sometimes pet store workers can have ideas too, but it's really, really hit or miss if they know what they are talking about, since many stores, especially big chains, don't train them much, if at all. Sometimes too, if you bought your axie from a breeder, you can contact them for ideas. But if you don't have a vet in your area and know of a store, especially like a family run one that sells axolotls or something, they might have ideas.)

If I may suggest, it might be a good idea to keep a diary, ideally with photos, of your axolotl's eating habits and the spot/any spots on them, to keep an eye on it and monitor any changes. This would be helpful to you or a veterinarian in diagnosing the problem.

Hopefully it is not this: http://www.caudata.org/forum/f46-be...lanoma-cancer-axolotls-reference-picture.html

But think about your options in case it is something like melanoma. Is there a qualified vet available and can you afford that sort of treatment? If so, you probably want him seen sooner than later, to either rule out melanoma or to get it treated.