Yes, that's why you're having nitrate problems. It should be zero. You have been removing dirty water from your tank, and replacing it with water that is just as dirty, or worse.
I suggest stop paying them for dirty water and start making your own. That way you know that it is clean. You should also tell them that they need to change their filters. They are charging customers for bad water! It wouldn't hurt to have them test their own water too, to make sure your test kit is good.