[ChuckG]

The Hanna meter IS working properly. I didn't specifically call Hanna to see if there is a way to calibrate because "I think" you sort of calibrate it for each and every test.

You take your sample water and have the meter ZERO it (isn't this like calibrating it?).
Then you add the reagent and let the meter do its thing and it gives you a reading.

With some more tests under my belt I have come to the conclusion that it is working and working very accurately.

Originally I got a reading of .01 on both of my tanks as well as my RO/DI water. The RO/DI water came from my holding tank.

I have since tested someone else's tank and got a reading of 1.00 - ouch! Yes, he does have algae growing.

I tested fresh water coming directly out of the DI chamber of my water filter and finally got a reading of 0.00. Retested the water from my RO/DI holding tank and still get 0.01.
Just prior to a water change my tank water was reading 0.03. After a water change it was back down to 0.01.

Now comes the part I don't understand. I'm obviously using water that has 0.01 PO4 to mix my salt water. I just tested the salt water in my holding tank and get a reading of 0.00.

????????????

I think this is enough to convince me that the meter works and I was assuming that I'm getting a little bit of PO4 leaching from the plastics in my holding tanks but now that I get a zero reading on my salt water holding tank I don't know what to think.