if your water has chloromines then unless you are using a dechlorinator that bind/detoxes ammonia then you will have traces of ammonia, also axolotls don't just poop they also urinate so they produce waste into the water even when it looks clean.
test for ammonia after 12 and 24 hrs as the tub can be to small and allowing ammonia build up.
also scrub the tub well when cleaning as bacteria build up in plastic tubs can cause rot and septicaemia.