Illness/Sickness: Is this "red leg"? Please help asap

[AbrahamAxolotl]

I've had him for over two years but have never seen anything like this.
For two days now he's been red and splotchy and it looks like his skin is bumpy. Like pimples. His tail and gills are curled.
He's not lethargic, he's still moving around, he ate a little last night (salmon pellets). And he passed a poop too but it looked darker and harder than normal.
I did a water test (twice to be sure) and the ammonia was at 0.50 so I did a water change (I do it once a week and it wasn't due until tomorrow but I did it again last night because I'm worried), nitrite is 0 and nitrates are orange on the chart.
Water temps controlled by chiller and are at 63F (double checked with glass thermometer). I've not added anything new to the tank in years or changed any products. The only thing new is a bag of new food bought from the same vendor as always (also a user here).
Please help, idk how to help him. I was thinking bad constipation but I'm worried it's more serious.

 

[xxianxx]

Six days late but,...... You had an ammonia spike, monitor your water quality a bit more, increase water changes​ if necessary. Your axolotl is ammonia burnt, it needs clean water to recover. I would keep it in a tub till the tanks def ok, watch out for fungal infections.