So what do corals actually want ? With such heated debates raging over ideal Po4 levels , where does it leave the layman reefer like myself ? With the realization that corals are highly adaptable ! But looking at what's best for beautifully coloured SPS a logical argument is the following , low , not zero but low dissolved inorganic phosphate levels , meaning the kinda po4 you testing for . Inorganic means there is no carbon molecule involved , and hence organic means , there is a carbon molecule somewhere in the mix . So don't confuse this with the carbon you might be dosing to reduce phosphate ... So what is low and why do so many sps-heads swear that zero phosphates are important ? Consider testing formats first ... Hobby test kits are often faulted as being unreliable and have a reputation amongst " pro aquarist " for reading much higher than the actual value , "so some might say " , Oasis , 1997 ( date check ) that when using hobby based kits run it on zero because the kit is not sensitive enough to read low down enough to detect the level you actually wanted. Others would say avoid that drama and protect the system from falling too low or being too high by investing in an electronic phosphate meter . Then the hardcore users would say stuff all of the above and send your water off for ICP testing and get the scoop proper ! I say , take it easy brothers and sisters , smoke a cigarette and drink a cup of tea . If you a newbie , stick to a detectable level on a hobby based test kit , I've never seen sps die when po4 levels were elevated , even as high as 1ppm ! I've seen countless people crash their tanks at low po4 levels . 0.03 on a hobby based kit is a safe zone . As you progress and gain deeper insights into total phosphate management you can allow levels to drop further .
So why do corals crash at low phosphate levels ? Well seriously speaking I don't think anybody can answer this in its entirety , but in the absence of scientific findings until we have more pieces of the puzzle , let's do what the big boys do and speculate.
Zooxanthella are algae , algae need phosphates , low to zero phosphates make for sick to dying zooxanthella , dead zoos in the coral make the coral sick and taking into account this environment is teeming with bacteria you can imagine the knock off effects . We should also recall that corals ( at least in the wild ) are feeding constantly , taking in organic material containing organic phosphate , and after metabolizing their food produce waist that feed inorganic phosphate to the zoos . ( perhaps this is where some of the inboard bacteria come in ? They take the waist of the coral within the coral itself in an organic form and metabolize it further into inorganic constituents , as primary autotrophic bacteria do ,!and their activities by products stream inorganic phosphates to the zoos ? ) . This is how zooxanthella are able to remain productive in natural coral reefs where dissolved water born inorganic phosphates are typically super low .
So if we had two aquariums along side each other , and in the one the inorganic phosphate levels were just drummed down to zero we should see a collapse of the coral , possibly due to the collapse of its zooxanthella . If in the second aquarium , the same was the case but the corals were well fed we should not see the same occur , the coral should continue to be happy and healthy and possibly in a much better state . However zooxanthella need more than just phos to be groovey , light and the rest of the inorganic constituents need to be present too , with good , healthy light and the correct representation of all required elements will come the making of good healthy zooxanthella. Healthy , low zoos levels mean less brown mass within the corals and therefore the corals can appear more colorful .
So ... Testing your phosphate is not the be all and end all of all . And a zero detectable phosphate level simply means there is no detectable inorganic phosphates in the water column , it certainly does not mean there is no phosphate , if this was the case , life in the aquarium would be undetectable too . While I have not touched on ( simply because I don't have a clue ) the possible effect of coral directly absorbing phosphates from the water through translocation , to feed or stream through to the zoos , we can assume that coral feeding is a source of phosphates to the zoos and this can account for productive zoos in oligotrophic conditions.