Thanks for your time, Roy and Diana.
Finally, I realized that I mistook to the instructions on the API test kit. I was accidentally expecting the water to turn from yellow to blue, instead of blue to yellow... Oops. This is pretty ridiculous especially considering I have run this test quite a few times... User error. Maybe it's having two kids under 2 that is sapping my intellect. Anyway, Now it all makes sense. The KH in the STS water is zero, hence why I am never seeing any blue in the test liquid. The STS is extracting the KH as would be expected...
Now I added a small bag of crushed coral to the filter in the STS tank. I am thinking that this carbonate buffer will "feed" the STS until it eventually stops pulling it out of the water...
Great stuff, BTW, STS. I followed your advice and went through W.W. Granger to get a bag locally in Portland. Thanks for the tip! Very easy to work with, looks great, cleared up fast, and so economical...
Here is a pic of the STS tank:
