I had the same problem when I first started a SW
aquarium, after I added my Protein Skimmer my nitrates dropped big time. I am currently running nitrates below 5 ppm
with monthly water changes of 20%. My tank is very lightly stocked however, so this could be a big part of it.
I recently had to add a UV
sterlizer too, I don't think I will do saltwater again without these two wonderful pieces of equipment. BTW
, I got a SeaClone 100, which I only recommend if you have a sump, cause it didn't work as a hangon very well at all, but works like a champ in my sump!!!
Good luck, keep us posted, and I'm sure someone will post with a better answer than mine as well!!!