In an established tank, I don't test my water unless my fish are acting strange or I'm having some sort of problem.
You don't put in something to get rid of the nitrates, you need to do water changes to bring them back down into a safe range (no higher than 40, preferably right at 20). You can do several water changes a day (with a good dechlorinator) until you get things under control. Make sure to vacuum the substrate well to remove any excess waste.
It would also be a good idea to make sure your filtration is in good running order and not too "gunked" up. (Rinse in tank water...)
What is your normal tank maintenance routine on this tank?
What size tank is it? How many fish?
How often are you feeding? How long does it take the fish to consume what you offer? What are you feeding?