I don't think it is your filter as someone suggested. Filters don't filter out nitrate anyways, they create nitrate by converting ammonia to nitrite then to nitrate. So if anything, it shows you filter is doing it's job.
I think it may be nitrates in your source water. Do a full water test (ammonia, nitrite, nitrate) on your source water that you use for that tank. Odds are it may be going in already high, then your filter and beneficial bacteria are adding to that number to make it go even higher.
Another thought, maybe there is something hidden that is dead/decaying that you are not seeing, like some dead shrimp hidden in foliage of the plants. Maybe something under the substrate that is leaching out into the water like dead plants roots.
Last suggestion would be to have your water tested at LFS to see what their tests say. Maybe you have a bad test kit for whatever reason. I know you said it wasn't expired. It could have been contaminated or compromised some other way, who knows. Good to have an outside source check to see if they get same results. Maybe bring a sample of your source water and one of your tank water. Petsmart and Petco along with other places will test your water for free.
I had thought the something about the source water, but I use ro water, and then remineralize it with kent ro right, I tested the water before and after re mineralization and all readings came out to 0ppm for nitrate nitrite and ammonia, ph at 6.4 wich is what I kee my tank at anyways. So it can't be the source water, the only possible thing I can think of is that the substrate itself is leaching out nitrate somehow.
My tests do not expire until 2018 so it sais on the bottle,and I had just bought it onto long ago,I would rather rely on a test kit than the strips that petsmart uses. I work at petsmart and even I test customers that its a waste of money to buy the strips because they are highly inaccurate.
I just did another 50% water change and tested water of tank and source water before I changed water. Tankis lower at 80 ppm since I replaced the purigen. And since the source water has no nitrates I'm hoping that in a couple hours when I do mother water change it will red out below 40ppm. I may change 25% more if it is at 40ppm just to keep it low.
Should I do more than one water change a week. Perhaps 15% twice a week