Axolotl's losing feathers on gills

I have 2 axolotls. 1 of them has lost feathers on some of it's gills. I am not sure why. The temp is 65 degrees Fahrenheit with the chiller. The water parameters are:

Ammonia:0
Nitrite:0
Nitrare: 20.

PS:He is still eating and very active. Someone help please,

Could you post a picture? What is your setup like?

How old are they?

'Very active' isn't always a good sign; adult axolotls are quite lazy.

I don't know how old he is but he's a little over 6 inches. The tank is bare bottom. With 3 decorations, an internal filter and a submersible water pump for the chiller. The tank is a 20 gallon long. N when I say active I mean that when he sees me he swims to the top of the tank because he associates me with food. I've attached a picture. It's the gill on the right with a white tip where the feathers are gone.

I had this problem too and found a couple reasons for it
First, the intake for my filter inlet had plastic slits to keep things that are too big for it from going through, i found my axolotl stuck to it by the suction, once time he actualy got a gill stuck in and a small piece got ripped off
I had to put some filter media over it to keep that from happening
The other time i noticed it happening I had neglected my water changes for a couple of weeks
The water still looked clear and tested clean, similar to your, but his gills still seemed to whither and shrink over several days
I started doing bi-weekly (twice a week) water changes and his gill have grown back very nicely

check for anything the gills may be getting caught on, watch him while he is active and see if the axie gets caught on anything
How often do you change your water?

I do water changes weekly. They do tend to hang out by the internal filter and water pump. I'll try do water changes twice a week and see if I see improvement out of the one losing his feathers.