Brass tarnishing in ultrasonic

I usually polish my brass with fabulustre and then ultrasonic in a
bath of Mr. Clean. I have tried palmolive soap and a compound sold to
me by my jewellery supply store. I have found that my brass tarnishes
in the ultrasonic bath if I leave it two long (read five minutes,)
with all of these solutions. Mr. clean is the best so far, but it
still tarnishes if I leave it to long.

I am trying to figure out what could be happening. I thought maybe
it was the fabulustre residue tarnishing the brass, but it is listed
as being appropriate for brass so that seems unlikely. Perhaps its
the Toronto water supply. I’d be interested to hear if anyone has had
similar experiences and knows why this might be happening.