I had a problem with nitrates and switched test kits. Was using the quick test strips that you dip in the water. Nitrates were always high. Spent a little more on a test kit that took a little more time and Nitrates were actually almost zero.
I've learned to feed less. My fish look pathetic coming to the tank all the time, but there is less waste, even if it looks like they get it all.
I had a problem with nitrates when I would get ocean saltwater and didn't use it right away. There would be some die off and I wasn't able to get my nitrates down. Now I'm going with strict RO
/DO saltwater. I'm considering mixing my own, but I'm not that brave yet.