One thing to keep in mind, is that due to the fact that you are testing saltwater, you will always get a certain level of voltage reading. It has something to do with the ionic nature of saltwater, and the electrochemical reactions between it and your probes.
I just tested my tank, and I'm getting a reading of between 5 and 33 volts. The interesting thing is, that the reading changes depending on how much flow is going accross the testing probe.
If I shut all pumps off, and test by inserting the positive probe into the saltwater, I'll get a reading of about 5 volts. If I move the probe around in a stirring motion, it will jump up to 12 volts.
I'm still trying to figure out how much is common to expect when testing this way. I still haven't got it figured out.