I couldn't get rid of it . 25% water changes three days in a row. Then every four days .
Run GFO !!
Wait .. Wait ... Each morning you should see a little less keep fighting until you see none for a week
Also I purged my sand and rocks with a turkey baster and made a mess and kept doing water changes like that . Nitrates hit zero fast. Phosphates hit zero.
Buy some snails that move the bed