But that’s why you have to get the water tested! There might be high levels of DHMO in it, after all…
One of my co-workers claims that he saw a report on the local news that this scam has been going around. While one person is testing your water, another is rifling through your stuff.
I haven’t been able to find that online anywhere, so I don’t know if this is true. Since I am not a tidy man, they would have a hard time rifling through my stuff and finding anything of interest… 
Ah, yes, the old “home water auditing” scam.
“Good evening. Have you accepted the Culligan Man as your personal savior?”
Guy came to our door, I gave him a sample from our 55gal aquarium - when he called back to discuss it - I told him to mail me the info - he never called again.