Illness/Sickness: Fungus? Stress?

Hello all,
My little guy is not doing so well. A while back I picked up my second axolotl from a LFS because they were housing him with lots of other fish in and 80 degree tank with a gravel bottom. Recently (after some fridging and close care) I believe he has passed all of the gravel that he had eaten while he was in there..
Since then he has been eating more but is not very active. he hides most of the day and only comes out for feedings.
Today I noticed that his stomach is a much lighter color than before. He was a very dark grey color when I got him but he has since lightened up some. Then I noticed there was a really light colored spot on one of his gills. not like a fuzz like I see people talk about a lot but the skin looks light colored.
When we got him his gills were in really bad shape and I almost bought another from the store today because they had him in the same conditions but I cannot afford another tank right now.
Is the light coloring a fungus? If so should I administer Salt baths? I will attempt to get some pictures tomorrow.

As for the grey coloration at the stomach, that probably was the gravel that you mentioned he passed. As for the white spot, I do not know yet, pictures would be helpful. In the meantime though, you could google what fungus looks like on axolotls.