I've been using prime since the beginning and haven't had high values like that. I am properly following the directions and shaking for the time directed. I have two API test kits and both expire on 2/26. My new tank (shrimp only 10G) doesn't have the same problem, so I don't think its the water...
I have a 55G tank with plants and a bunch of shrimp and fish. All my water values were always stable, but I had to put in some Nitrofurazone and API Pimafix (separately, about a week apart) to try to fix some cotton mouth on a few of my neon tetras.
Now I constantly have high nitrate values...