Nitrate limitation is the term used by advanced aquascapers to describe the strategy of unlocking the deeper reds in selected plant species that respond well to those conditions.
Nitrogen is the element that is used in the largest quantity in plant tissue besides Carbon/H2O. For many natural ecosystems the availability of nitrogen determines the overall growth rate of plants. Nitrogen rich soil is great for farming for example.
For many aquatic plants, chlorophyll development is delayed when there is a lack of Nitrogen available - under this condition this can make plants that are red or orange appear significantly redder. For species such as Rotala rotundifolia and its variants colorata/H'ra/etc, Rotala Goias, Hygrophila pinnatifida & H. araguaia, Ludwigia arcuata & L. brevipes - they only get truly deep red under very low N conditions - using high light alone will not achieve the same effect.
Rotala rotundifolia red; you can tell exactly at which point in its growth cycle that Nitrate limitation occurred in this tank; where the leaves start growing much redder.
A side by side comparison of Rotala H'ra grown in two different nutrient settings:
Nitrate limitation has no impact on coloration for many other red plants such as Ludwiga sp. red (above in pic) and Alternanthera reineckii (bellow)
Ludwigia arcuata grows with narrower, redder leaves under nitrate limitation. When NO3 is rich, it grows wider leaves, in a more pale yellow.
This effect is achieved when all other nutrients are available in excess, but Nitrogen is supplemented at a controlled rate. This approach is often done by dosing less N in the water column, but using a rich substrate, which serves as a backup store for N. We do not want N to bottom out completely as this leads to stunting/stop of growth.
The other advantage of using controlled Nitrogen levels is that it gives tighter growth forms and shorter inter-node distance for some stem plants. This is particularly desirable for the aquascaping crowd which want to control plant bushes by trimming to match hardscape contours. Slowing down growth rates often also mean more stability for the tank overall, as there is less trimming/replanting, slower overcrowding - which are very common issues for aquarists.
If you are intending to farm plants at maximum speed, high N levels will achieve that goal. Whereas if your goal is aquascaping and controlling the aesthetic sizing and form of plants, and deep reds for coloured plants, going the N limited route will be more advantageous.
Hygrophila pinnatifida requires Nitrate limitation to get red. Even in bright light, the leaves will be olive green if NO3 levels are rich.
In terms of the Capstone range of fertilizers, the benefits of nitrate limitation are realised when paired with the right tank environment: