Let me first say that I know only slightly more than zero about reef chemistry. With that said, I would try a huge water change (90%), refill, re-dose the ammonium chloride and test again over a couple of weeks.
The reason I say that is removing the water will remove anything in it that might be consuming the ammonia directly; it shouldn't affect much of the bacteria that will already be in there as that will be mostly in your rock/substrate. I have no idea what is in XLM but I wouldn't add any more at the moment, in case that is skewing your test results.
I also don't 'wash' the test vials. I rinse them in tap water, then RO and then let them dry. However, if you are doing the same thing on your other tank then that's probably not a factor.
To me, the only difference between your tanks appears to be Dr Tims v XLM.
The reason I say that is removing the water will remove anything in it that might be consuming the ammonia directly; it shouldn't affect much of the bacteria that will already be in there as that will be mostly in your rock/substrate. I have no idea what is in XLM but I wouldn't add any more at the moment, in case that is skewing your test results.
I also don't 'wash' the test vials. I rinse them in tap water, then RO and then let them dry. However, if you are doing the same thing on your other tank then that's probably not a factor.
To me, the only difference between your tanks appears to be Dr Tims v XLM.
