Hello, i would suggest minimal feedings for the time being. Very smLl amount every other day for the time being. Its kind if easy to get the nitrates down from 80 to 20 but 20 to 0 is really the hard part. You can get through this. Been there trust me.
Few questions... You said youve been doing 10g water changes and have seen no difference? How many days have you done this. There must be a source for the nitrates. Are you adding any chemicals or additives? If you are i would stop. If you are not using RODI
water, test the tap to see the nitrate level of it.
What are you using as filtration? Do you have a protein skimmer in your sump? Any bioballs or rubble rock? Any chaeto?