You need to compare your tank nitrates to your tap water nitrates, no amount of water changes will take your tank nitrates below tap water levels, its not at all unheard of for people to have high tap water nitrates. 15% water changes wont reduce your nitrate levels by huge amounts lots of small water changes has less effect than a few big water changes. as the results of successive water changes are diluted. For example if you had 100ppm tank nitrates and 0 ppm tap water nitrates you do one 50% water change your nitrates above tap water level will literally half you would end up with 50ppm nitrates inone water change. If you did 2 x 15% water changes followed by two 10% water changs you would end up with 58.5 ppm of nitrates. you will ahve changed the same number of litres of water but each successive water change dilutes the effects of the next one. On the same tank following the 50% water change the second 50% water change would not take nitrates down to 0, but to just 25ppm!
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10ppm isnt a high reading for nitrates at all many places have 40ppm from the tap! im lucky living in a 5ppm area.