For those that like details...
One way to view nutrients is "flow" (like a river) instead of "level". By keeping a very high "flow" of nutrients, by providing both a heavy input of nutrition and a heavy export of nutrients, it's like a fast flowing but shallow river. This type of flow is how natural water systems work (reef, lake, etc). They might measure "low" nutrients, but their internal flow of nutrition-to-nutrients is high; so high it's hard to believe. Too bad it's invisible. But you have to have both; if a river has less input, it runs dry, and if it has less export, it overflows.
One common situation that most people have seen is when nutrients measure low or zero, but the glass needs cleaning very often or daily. Food in the display is being converted to ammonia, and the ammonia swirls around and hits the glass before any filtration occurs. So the algae grows on the glass easily. The solution to this is to filter more so the ammonia in the display is lower, so when more ammonia is added, the total amount will still be too low to feed algae on the glass. Besides typical filtration, the best way to get this extra filtering is with more periphyton on the rocks, and this just requires more established rocks. Leaving the back uncleaned will also help.
A fast flow of nutrition-to-nutrients is capable of filling up voids quickly when needed, like if you put a bucket down into the flowing river. This void can be coral growth, too. So by inputting more nutrition (feeding, dosing, etc) and exporting more nutrients (macro growth, etc), the flow is large and available for corals to dip into, even though it might measure zero.
This may be one reason why some people say they get better coral growth when nutrients are slightly above zero; because there is not enough flow of nutrition-to-nutrients (input to export) when the measurements are zero, and thus the corals don't have enough of a river to pull from. In other words, the system is sort of stagnant, nutrient wise.
