Illness/Sickness: Axolotl Shedding Slime Coat/Decreased Appetite?

I did my axolotl's normal weekly water change yesterday and shortly after I noticed what I believe is the slime coat around the gill area coming off, almost like shedding. It was very soon after the water change which has me a little stumped since I do age and use Prime conditioner for my water. The skin all came off the gills within a few hours and he does look quite normal now. On top of that I have noticed a decrease in his appetite recently, he has still been eating but he is usually an absolute monster eater and wants to eat constantly, whereas lately its been taking some convincing to get him to eat. These two things have me worried and I'm unsure exactly how to go forward and don't want to do anything terribly drastic at this point since I dont have a strong understanding of what/if anything is wrong. He is about 3 years old now. He is a cherished member of my family and it has been nothing but a pleasure caring for him, so I would hate for anything to happen to him. I'll attach pictures the best I can with my setup listed below. I appreciate any help, thanks!

Tank Setup/Parameters

Tank Size: 10 Gallon
Ammonia: 0 ppm
Nitrite: 0 ppm
Nitrite: 10-20 ppm
Temperature: 65 F/18 C, regulated by a chiller
Water Changes: 30% Weekly
Staple Diet: Nightcrawlers