Are you experiencing algae issues? If not I would worry about dropping either levels any lower. Stick with the current dose as a maintenance dose.
Rocks will not soak nitrates and consequently leach them back. The substrate could be trapping nuisance nutrients but if you or the livestock haven't been disturbing it then rule that out as a source. Although it might not be a bad idea to vacuum it during water changes if possible. Not entirely necessary but it does help to keep clean.
A large water change can also quickly reduce nitrate but at 5ppm it takes a 50% change to get to 2.5 (not worth it imo).
If you don't have signs of algae I refer you to my opening statement.
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90g Mixed Reef
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