Don't wash or clean your tank, you will invariably muck up any established bacteria which is needed for your tank cleaning.
A thermometer is one of the most important things to have if you are keeping axolotls; so my advice get a thermometer and put it in your tank. It may very well be heat related stress. Cleaning and washing out your tank will do nothing but invariably harm your axie, as you will end up starting a new cycle to establish the good bacteria you've cleaned out - this can take 3-8 weeks to cycle.
So, buy a thermometer and don't clean your tank. Take a sample of tankwater to the petshop/aquarium as advised by the others and get them to test for ammonia, nitrite and nitrate. Get them to write the results/figures down and come back here if you're not sure what they mean.