Well, not overstocked.
I agree, keep with the water changes until it comes down. Also run some fresh GAC
To help with the long term, you sure could use some more rock, another 100 lbs or so would be great.
Also upgrading to a better skimmer.
Been set up for a while? Maybe your still cycling a bit. The lack of nitrate is a possible indicator. Have you ever had tracable nitrate?
Also, may want to take a sample to your LFS
for comparison. These test kits are not always accurate.
Your stock is not stressing at all is it, color and eating habits good?