The process is ammonia (wastes), to nitrites, to nitrates. Having a good mechanical filter to filter out solid wastes and a good biofilter with a good colony of beneficial bacteria (often within the same filter) can assist in the conversion. You are right, nitrates are the final step in which we need to assist with removal. The question is why are they so high when you're working so hard at doing water changes. Unless perhaps too many beneficial bacteria are also being removed in the process. Only change the nonbiofilter pads. Most filters have a solid waste collecting filter pad in which the water passes through first and then a biofilter pad where the good bacteria thrive, so the water can pass through where they can assist in converting the nitrites to nitrates. The beneficial bacteria also live throughout the gravel, once established.

So, the question is, in an established tank that should have a good colony of assisting bacteria, with a filter, and water changes, why are you having such high levels?