your nitrates are so high because a 10% and 20% water change alternating weekly are simply not enough to maintain the tank....you should be doing at least 25%-35% weekly ....if you still have high nitrates right before doing your next water change (that is, after having done 30% water changes weekly for a couple weeks), then you may have to up the weekly water change amount closer to 50% ....TBH, although it is not recommended i do around a 50% water change (or a little more) weekly on my largest tank due to a farily high stocking level