If you think about it, reducing nutrients via water changes can be difficult pending the size of your set up. If you have a 100 gallon tank and change 50 gallons of water, in theory, you reduce your nitrates from 100 to 50. A 10% water change is barely keeping up with the rate that nitrates are being produced, based on your post. If this is a new set up, the nitrates should begin to drop naturally, unless the rock and sand is saturated.
Otherwise, I would plan to start doing 25% or greater water changes 1-2X per week (a smaller tank will make this easier). You can also run nitrate binding media in a reactor or holding chamber that forces water through it at a low volume.