Hi there,
Have a look at this list.
http://www.caudata.org/axolotl-sanctuary/vets.shtml
So far you are doing well with ensuring good water parameters and temperature. Do you also test for nitrites and nitrates? What are you feeding the axie?
There are a few differentials i can think of.
1 - A bacterial scepticaemia such as aeromonas or chlamydophila infection.
2 - A platelet clotting defect (petechiation)
3 - Hypersensitivity to substance (metals, plastics, chemicals)
I would like you to handle the axie very carefully and take not to wash your hands with soap. Again, isolating the axie is best.
Here are a few things you can try.
1 - Use (or replace) activated carbon and zeolite granules in your filter for chemical filtration. Sometimes, our water supply comes 'contaminated' with certain chemicals that we may not detect just based on the water parameter test kit. The activated carbon will remove trace chemicals, phenols and organic debris whereas the zeolite will rid harmful nitrogenous buildup. If your axie has a hypersensitivity issue, hopefully this would help resolve.
2 - Sometimes, malnutrition and toxicity can cause clotting defects. Chemical filtration can help if trace toxins are in the tank. A good diet of live earthworms and bloodworms can support good circulatory health.
3 - Fridging. That would impede any scepticaemia cases and shunt the blood away from the peripheral vessels. It is palliative but not curative.
http://www.caudata.org/axolotl-sanctuary/Fridging.shtml
Cheers