If you’re having trouble with low nitrates may I suggest adding more fish to increase nitrate production? You can also manually dose potassium nitrate or sodium nitrate. There’s an online calculator for dosing nitrates that works well. I dose potassium nitrate whenever my nitrates get too close to 0-5ppm. I keep mine at 10ppm. I have a lot of corals an not many fish so I I have the same issue. Problem with nitrates at 0ppm is some of your coral will start to starve.
Here’s the calculator if you decide to dose. Search reef2reef for threads on dosing sodium and potassium nitrate if you’d like more info.
http://www.theplantedtank.co.uk/calculator.htm