PLEASE HELP! My axolotl carlos' belly is really big! He can't swim to the top of the tank!?

Please help me! I feed him fish flakes by hand. He's in a 20 gallon tank and he's like 4-5 inches. I really don't think he ingested the gravel because his mouth is so small. He can't even reach the top of the tank. :-( I WILL GIVE BEST ANSWER I'M DESPERATE!!!