My own experience has been that new tanks are pretty devoid of nutrients for plants usually and could use some dosing, even if they are low-tech, but it needs to be smaller dosing than normally recommended on the fertilizer and it can be phased out after the tank has a pretty full population of fish and is pretty mature. Tom Barr recommended this and called it "reduced EI."
Speaking of EI (estimative index), one of the important parts of it is the weekly water change and it sounds like your nitrate(NO3) of 40 may be telling you that the weekly water change will do better being a bit on the larger percentage size. I'm assuming your tap nitrate(NO3) is zero and if so then you will want to see if you can find a pattern that will keep the NO3 lower. Being at 40 is not a particularly bad thing though and it may very well be that the upcoming gravel-clean-water-changes will clear it out and it will settle at a lower level.
WD