Nitrates always high


Reefing newb
My pufferfish died 3 weeks ago. After testing i found nitrates were extremely high around 120 ppm. I have a 75 gallon tank i’ve been doing 20 gallon water changes each week. Doesn’t even lower the nitrates one bit. I’ve been using prime and nitrates haven’t been lowered one bit. I’ve cleaned the gravel and done everything. I’ve used other websites to see how to lower down nitrates and they said i need a saltwater expert to figure this out.