Nitrates over 60 impact immunity and over 200 shut it down. Anything in between has varying levels of impact and is rather species dependent as to what that impact is. I have never seen a study that tests the impacts on hemoglobin effects at levels under 200ppm which makes me think there isn't much of an impact under 200 in fish (not the case for mammals!)
If you did a few BIG water changes 75%, say 3 days apart, I am guessing your nitrates will be down to 50ppm by the third one. Once you have them that low measure how fast they come back up, and be ready to do a 50% water change when they hit 60 again.
With the nitrates at 60ppm, a 50% water change with the new water at 40ppm will put your tank at 50ppm. A 75% change would put it at 45ppm.
With the nitrates at 80ppm, a 50% water change with the new water at 40ppm will put your tank at 60ppm. A 75% change will put them at 50ppm.
Here's how to do the math:
What percent you change multiplied times the 40ppm in the tap ex: 50% x 40= 20
What percent you didn't change times what your previous level was 50% x 80 = 40
Add the two numbers up and that is your tank level. In this example 60ppm.
You really should try to keep your levels under 60ppm. Other than LOTs of water changes, using RO water mixed with your tap will lower the nitrates of your tap by a LOT. If you can buy RO water reasonably cheap and do 1 water change a month with a 50/50 RO/ tap mix you would be able to do fewer water changes for the rest of the month.