Axolotl not growing

Hello. I purchased two Axolotls about 2 months ago. One of them is thriving and has grown substantially. The other one has not grown at all. I think it has some kind of genetic defect. It eats well but still does not grow. Not even his little gills are growing. I try to feed him daily with frozen brine shrimp, bloodworms, pellets and very small pieces of live trout worms.

This morning I saw him/her kind of swimming around the top of the water gulping air every few seconds. Now it is kind of floating upside down a bit. Having a hard time staying on the bottom.

They are in a 20 gallon Long tank. I change the water often and keep the temp as low as I can. The tank is on the floor and I have a cooling fan blowing down on it.

I read about putting them in the refrigerator but I am afraid to hurt him. I think I bought a sickly axolotl. I have attached photo so you can see the difference between the two of them. When I bought them there was only a one inch difference in length. Now there is a 4 inch difference.

Any suggestions would be greatly appreciated!