you just need to keep changing water until you get trates down.
Larger changes will be more effective then small ones.
Have you considered carbon dosing?
I use 'bio plastics' I my 75(just started them again last week) to control nitrates.
Takes about a month to 'cycle in' .
I vodka dose my 120 and cycled for that in under a month.
With the vodka dosing I can control where I want nitrates.
The bio plastics will just keep removing nitrates.
Both methods need to have a protein skimmer running well.
Neither of those are going to help you now though.
Math says if you change75% you would still have at leas t40ppm trates and that accounts for 160 being accurate and not just the highest the test goes).
Since you changed more then 50% already I think your trates are over 160(considerably?).