no3 is the end result of the nitrogen cycle and is harmful to fish at 80+ ppm. It's more harmful to mobile inverts at 40+ ppm and harmful to corals/sea stars at 20+ ppm. Reducing the no3 level to 20 ppm or less within the next week or two is critical for the tanks health.
Most no3 test kits only test up to 80 ppm and if you are still showing 80 ppm after the 40% and 20% pwc then it was considerable higher then 80 ppm to begin with.
Need more info:
Size of tank?
How many fish and their sizes?
How much base/lr is in the tank (lbs)
Is this a reef tank with corals or fo/fowlr?
Is there a clean up crew? (snails, hermits, shrimp, ect...)
Is there a sump and if so how large.
Doing large pwc (50%) with no3 free water is the only way to reduce no3 quickly.
I don't like doing large pwc on tanks with inverts/coral/sea stars due to the risk of osmotic shock due to possible large swings in ph/sg/temp. If your tank does have any inverts/coral/sea stars then I'd recommend doing no more then 30% pwc every other day until no3 is reduced to 20 ppm or less. The other option is to remove them to a clean plastic
bucket and drip acclimate them for a couple of hours to the tanks new parameters. (don't expose sea stars/corals to air)
If the tank is fo without inverts you could do pwc as large as 70% but it's still risky unless you have a refractometer to very accurately measure sg levels in the pwc water to match the tank.
Either way you still want to mix your pwc for 24+ hours with a strong ph or pump and match the sg/ph/temp as closely as possible to the main tank.
Good luck.