Axolotl with white gills! Help!!!

[Silver5101]

I hope the photo has attached, please help me! I know there is salt bathing. My parameters were tested 0 ammonia 0 nitrite 40 nitrate. My other two are fine in their tank. What should I do please help me. Fizz is eating fine but those two spots on his gills are there and I'm very worried about my lil bub.

 

[Boomsloth]

What's the temp? Try lowering it a bit

 

[Silver5101]

The temp is already at 63 and I don't think it's going any lower nor does it need to?