Ok, everyone is jumping around to very different conclusions, all dealing with the tank water.
To be frank, you're more than likely doing the test incorrectly, or are not getting all of the reagent powder into the vial, or are not mixing the reagent thoroughly enough, or are not letting every single bubble in the reagent vial dissipate. All of these can separately cause inaccurate readings, any combination will cause incredibly inaccurate and inconsistent results.
To get accurate results, you need to use both vials. Fill both with 10mL of water. Now take your reagent packet and tap into one corner. Cut the packet in half diagonally, and then push those two tips together, and you'll make a little pouring spout of sorts. Pour the reagent into one vial, put the cap on, and shake it like mad for at least a minute, then LET IT SIT until it is perfectly clear, with no reagent undissolved on the bottom, and NO BUBBLES in the vial. Then turn the colorimeter on, wipe the first vial (with only tank water in it) down with a towel to remove any fingerprints, and put it in. Push the button, and when it says C2, put the vial with the reagent and water in (wiping it down first to remove fingerprints, etc), and HOLD THE BUTTON. This will enable a 3 minute countdown, in which it will test after the 3 minutes. This is to let any more air bubbles dissipate, just in case any were missed before. Once it is done, it will do the test and give you the readout. If you followed those steps, then you will have the most accurate results possible with this meter.