One of our two axolotls seems to have a growth uder his chin - it is as though his head weighed twice as much as it should and he has difficulty propelling himself around. He has also developed grey freckles on his face and his gills are a much darker red than they used to be.. He is still eating (bloodworms), but sometimes shoots up to the top of the aquarium (to get a giulp of air ?). I have checked water quality for PH, ammonia, nitrates etc - all OK. Any ideas?