Here is a good read:
http://reefkeeping.com/issues/2006-09/rhf/index.php
And another:
http://www.advancedaquarist.com/2002/4/chemistry
IIRC noticeable inhibition of calcification starts around .01ppm PO4 and the inhibition drastically increases as PO4 levels rise. In layman's terms, the more phosphate you have in your system the slower your stony corals will grow.
The train of thought is with zoanthids softies and some LPS is that they prefer "dirtier" water. IMO I think most confuse dirty with elevated nutrient levels where higher NO3 & PO4 are generally acceptable. The truth of the matter is that these organisms can tolerate higher levels but I believe they don't necessarily thrive or require this level. I believe dirty water needs to be looked at as having a higher DOC and/or a healthy microfauna population, not increased nutrient levels. Also, like anything saltwater related these animals are very sensitive to change and if they are used to being in elevated nutrient levels then a sudden drop can cause harm, hence why it is recommended to slowly start GFO and ramp up gradually so you dont drop your levels too fast. I am not aware of any studies that show a need for a PO4 level greater than .01 for reef invertebrates, if there is one I would love to read it
IME the most colorful reef home aquaria I have seen keep their nutrient levels unmeasurable. Not only does this keep the zooxanthellae in check it also helps keep nuisance algae at bay. I have also witnessed this with my home aquarium. When my PO4 levels rise above .03-.04 I notice my SPS starting to brown out.
To answer your question coloagro - I change out my GFO every 2 weeks. After 1 week I get a 0 reading for PO4. After week 2 I strive for it to be .001-.02 but sometimes it jumps up to .03 if I am not consistent with my feedings or I just trimmed a bunch of macros back. I only care to have a reading >0 because of my macro algaes. If I didn't have macros I would probably use more GFO at a time, change it out less and strive to keep my PO4 0 at all times. When my PO4 bottoms out I have not noticed any ill effects with any of my corals and my zoas grow just fine.
btw, if you are using GFO as directed its almost impossible not to bottom out your phosphate unless you are really overfeeding or have a new system with Pukani rock which is known to leach phosphates into a system, then your GFO will exhaust very rapidly. GFO is VERY efficient at binding phosphate from the water column.
Just my 2 cents.