Why are my axolotl's gills degrading?

[EllieWind]

I could use a bit of advice on this as so far this has me completely stumped.

One of my axolotls, a 6 month old that I've raised from egg, has had its gill filaments essentially shriveling up and dying over the last month. I have the little guy in a 15 gallon with another juvenile of the same age and size (only temporary until I can move one of them into my 55 gallon tank), which I do 20-30% water changes on up to 2 times a week as needed. Parameters have consistently sat at:
ammonia 0ppm, nitrite 0ppm, nitrate 40ppm, with a ph of 8.0. Temperature usually sits at 65°, but I have been lowering it to 59° to aid my other axolotl which has been constipated (now fridged).

I can't find any reason for my little bean's gills to keep declining, at least as far as the tank's health goes. I've recently made some Holtfreter's solution and added a little to the tank in case something is up with our tap water, but it doesn't seem to be helping.

For reference none of my adult axolotls have been showing any issues with their gills, and one of them has just grown her gill fluff back beautifully after an infection had her under the weather. Can anyone help me out or give any insight?

Pictures below show this baby's gills a month ago and today.

 

[landonewts]

I am far from an expert, but they actually look good to me....? I know that my axolotls gills got less "floofy" as they got older, and your parameters sound great. I can tell you're taking great care of your axies. Maybe less is more otherwise? Hoping someone with more experience weighs in, I've kept newts for decades, but only got my two axolotls a year or so ago.

 

[Calgarycoppers]

Your nitrates are a bit high and that will cause gill damage

40ppm Nitrate should trigger an immediate minimum 50% water change - Personally I'd do 75% at 40ppm.

Try to plan for your nitrate to be 10-20ppm after your water change.

20-40ppm should be 33-75% water change.

Gills are more a result of water quality and hardness.

Adding salt, johns or holtfreters solution can help fluff them up along with keeping the nitrates down

I could use a bit of advice on this as so far this has me completely stumped.

One of my axolotls, a 6 month old that I've raised from egg, has had its gill filaments essentially shriveling up and dying over the last month. I have the little guy in a 15 gallon with another juvenile of the same age and size (only temporary until I can move one of them into my 55 gallon tank), which I do 20-30% water changes on up to 2 times a week as needed. Parameters have consistently sat at:
ammonia 0ppm, nitrite 0ppm, nitrate 40ppm, with a ph of 8.0. Temperature usually sits at 65°, but I have been lowering it to 59° to aid my other axolotl which has been constipated (now fridged).

I can't find any reason for my little bean's gills to keep declining, at least as far as the tank's health goes. I've recently made some Holtfreter's solution and added a little to the tank in case something is up with our tap water, but it doesn't seem to be helping.

For reference none of my adult axolotls have been showing any issues with their gills, and one of them has just grown her gill fluff back beautifully after an infection had her under the weather. Can anyone help me out or give any insight?

Pictures below show this baby's gills a month ago and today.

 

[EllieWind]

I did a water change right after, been trying to keep my nitrates down.