I run very clean tanks with low fish bioloads and often have trouble with low nitrate preferring to keep levels ideally around 5ppm for all corals You can only feed so much. It's also a lot easier adding nutrients with a low bioload then having the opposite condition.
I use potassium nitrate; basically stump remover. Works great.
I have the same issue with phosphate and use seachem flourish phosphate to keep those levels at a trace. Not convinced acropora need it, but montipora goes crazy when phosphate is kept present and stable.