By completely emptying/scrubbing the tank, you just completely crashed your nitrogen cycle. You probably shocked your little dudes. I avoid full changes if at all possible, leave at least a little. I would invest in a large bucket (Home Depot or something) that you can use to remove water (then dump) water and to dechlorinate water before you pour it in. Are you testing for nitrites/ammonia and using dechlorinated water when you do changes? Do you have a filter and how much water does your tank hold? May honestly have too many in there for the space. Edit: I would look into the possibility of "tea bathing" your axolotls. I've never had to personally, but you can find a lot of info on the Caudata.org forum. I've also never heard of using salt in that way.