Planted Tank Guru
Join Date: Jun 2004
Location: Baton Rouge, LA
First make sure you really have zero nitrate.
Add a single grain of dry nitrate to your test sample. Or the tiniest smear of your liquid nitrate solution. Or far better, look up instructions for calibrating a nitrate test, which involves preparing a solution of known concentration.
And test it. Still get zero? Your test is wrong. (Though it's also possible you or your supplier have mixed up nitrate with something else, and you're not dosing nitrate as you thought.)
You'll notice the test instructions say one bottle must be shaken. That's because the active ingredient is solid crystals in suspension. They need to be evenly distributed in the liquid for a good test result, but over time they tend fall to the bottom and clump together. A clump can also partially block the dropper, preventing any crystals from coming out whatsoever, but still allowing useless liquid to come out. All of which can lead to low or zero test results.
To fix the test, you have to whack the bottle repeatedly on a hard surface to break up clumps. Shake the dickens out of it. Possibly clear the dropper with a fine needle. And then when dispensing, notice if it's hard to squeeze drops out, or suddenly becomes harder partway through. If so, you've got another clog, stop immediately and beat/shake it some more before continuing. If you still can't get a nitrate reading with a sample known to have nitrate, the test may be genuinely bad - which is rare, but I have seen it. (All the info I'm sharing with you came to me courtesy of a chemist working for a test kit manufacturer, in diagnosing why a brand new test didn't work. Learning so much made up for the inconvenience!)
Things like this are why some folks say tests are unreliable. And often discourage their use entirely, which is silly in my opinion. They work fine once you learn to deal with their quirks.
As for your personal nitrate level, I know PPS-Pro doesn't recommend any specific level, or even testing for levels; but rather a dosage the tank is found to respond best to, as determined experimentally. While I haven't used PPS-Pro, I would not expect this to result in a level of zero! If it's truly zero, your plants may still appear to be growing well, but not as well as they could. I'd say 3-5ppm is a practical minimum, and optimal may be higher. Once you've established that your test is working properly (or even without), you can try dosing some more nitrate, and see what happens. When working without tests, I usually try a 50% dosage increase for two weeks, that's typically enough to see whether there is a positive effect.