One side of my axies gills are shorter than the other

[Critterkid]

I've had my baby axolotl for a little over a month now, he is about 3 inches long, eating great, and in a bare bottom tank all by himself. I've noticed that over the past week, the gills on his left side have started to get shorter an slightly less fluffy. They aren't curled and neither is his tail. The only water movement in this tank is a steady drip of a homemade chiller that keeps his tank around 15C. I tried taking pictures but the difference in size wouldn't show up good. As I said, he is eating and acting perfectly normal, and has been since I got him.
Thank you.