Many people have heard that reducing nutrients to "zero" (nitrate, phosphate, etc as measured by hobby test kits) is not good for coral growth. The answer is, it depends. "Zero" is not really zero of course; hobby test kits don't need to test super low levels like lab tests do. In other words, there is always a level of nutrients there, even if you have "all zeros".
First thing to know is that natural reefs always test "zero" for nutrients with hobby test kits. Yet, they have very high growth rates; acro's can grow two feet in one year. But yes, some hobbyists do see less growth with zero nutrients. Why is this, and what is different with tanks compared to natural reefs?
One answer is food particles. The enormous amount of visible food particles (and invisible dissolved particles) in natural reefs is so high that at night some divers can't see their own hands. And the enormous level of photosynthesis on reefs consumes most of the resulting nutrients. But hobbyists can't have this level of food particles because of nutrient build up, so they reduce feeding, to as little as 1/1000 of natural reef levels. Thus, corals can starve from lack of food particles.
So one bandaid to help with this is to keep tank nutrients above "zero". This feeds bacteria around the corals, and the bacteria then become coral food. Softies take it in directly, by inflating with water, and polyps eat the microbes and pods that eat the bacteria.
This works but is self-limiting, because in most cases higher levels of phosphate is going to slow the coral growth, and also because bacteria are hard for polyps to grab. So the trick is to keep food particle levels up high, and keep nutrients down low, at the same time. In other words, try to duplicate natural reefs.
Also, some people say they get some brown dust algae (usually dino's) when nutrients go to zero. However this is temporary, because of phosphate leaving the sand and rocks. After a few weeks, this will go away.
