Just because you CAN feed pellets to an axolotl and not kill it doesnt mean its a great, or even a good food. Try earthworms.
Find out what EXACTLY your parameters are. Saying that you are getting them professionally tested doesnt help if the person doesnt know what the proper parameters are for your pet, in this case an axolotl. Heck, YOU should come home with the knowledge of exactly what your parameters from that testing are. I would never leave the care of any of my creatures in someone elses hands entirely, and just be ok with hearing something along the lines of "oh, your temps are fine." Ok, but what are they! (snake example but I'm sure you see the connection).
If you do know what they are, then thats good, but stop doing the crazy 100% water changes. You are just going to crash your tank if you havent already. Once that happens, ammonia and other toxins build up pretty crazy fast, and you will pretty much have to get your water tested every day until your cycle completes again. Get a test kit and do this at home! And not those stupid dip-strips, I'd recommend the API freshwater Test Kit. I love mine, and I learned so much from testing my own water parameters.
Like mentioned earlier by another, making sure the water is the same (or very close to the same) temperature and PH when you do such gigantic changes is very important. I use a temp gun when I do large water changes (like while i was cycling my tank and trying to get my nitrites to drop), to make sure that my temp was the same. I knew the PH was the same because I tested both with my kit first.