Because it is a perfect analogy that shows the Redfield ratio (an uptake ratio, which is EXACTLY analogous to the uptake ratio of 18-20 ppm calcium for each 2.8 dKH of alkalinity) is NOT an aquarium target ratio and should not, buy itself, be used as such.. It is simply the ratio of elements that the organism uses.
The simple reason that the oceans don't need to have a "store" of 65dkh or 420ppm CO3 to match the 420ppm Ca is that calcium is an element/ion, the sea can not make this element from other elements, just as we can not turn lead into gold. Alkalinity is a compound/ion, it's made up from compounds and ions and elements and as such, the ocean can make this compound at will, 24/7 .... if you uptake it, more is quickly made, no need for nature to store it and nature keeps the levels balanced.
As reefers, we have a limited body of water to play with, so we often have to have elevated levels of some elements / compounds / ions etc to ensure that the important ones don't bottom out, and we typically have to dose these regularly as our tanks can not build or release what is needed themselves.
The fact that, in the article that you post, some parts of the ocean (not the part where our organisms live, by the way), happens to fall at ONE point on this ratio is meaningless (IMO). To say the ratio is desirable at any other point is just not right, as my extreme examples point out. If you want to quote concentrations in tropical ocean surface waters and give those as targets, that makes much more sense, but itself has limitations since we do not feed corals the same as they are fed in the ocean.
It is generally, in fact widely accepted, that there is a TYPICAL composition for sea water. Yes, of course real sea water all over the world and at various depths differs, but the typical quoted composition is;
<0.1 ppm Nitrate and 0.005ppm Phosphate , you even quote that yourself here >>>
Reef Aquarium Water Parameters by Randy Holmes-Farley - Reefkeeping.com http://www.reefkeeping.com/issues/2004-05/rhf/index.htm
16 x 0.005 = 0.08 == <0.1 are very close to redfield.
The problem for us reef hobbiests, is in being to accurately test for nitrates and phosphates, all test kits have margins of errors, and we humans are also prone to errors. But most kits can detect to 0.015ppm PO4 +/- 0.015ppm (Hanna ULN Phosphorous and Salifert are the kits I personally use). This means if we test for PO4 and read 0, then the PO4 is in the 0 to 0.015ppm range. If we read 0.015ppm PO4, its in the 0.03 to 0 range. So the best we can test down to is 0.015ppm PO4.
Taken from your article again ;
Nitrate
Nitrate is an ion that has long dogged aquarists. The nitrogen that forms it comes in with foods, and can, in many aquaria, raise nitrate enough to make it difficult to maintain natural levels. A decade or two ago, many aquarists performed water changes with nitrate reduction as one of their primary goals. Fortunately, we now have a large array of ways to keep nitrate in check, and modern aquaria suffer far less from elevated nitrate than did those in the past.
Nitrate is often associated with algae, and indeed the
growth of algae is often spurred by excess nutrients, including nitrate. Other potential aquarium pests, such as dinoflagellates, are also spurred by excess nitrate and other nutrients. Nitrate itself is not particularly toxic at the levels usually found in aquaria, at least as is so far known in the scientific literature. Nevertheless, elevated nitrate levels can excessively spur the
growth of zooxanthellae, which in turn can actually decrease the growth rate of their host coral.
For these reasons, most reef aquarists strive to keep nitrate levels down.
A good target is less than 0.2 ppm nitrate. Reef aquaria can function acceptably at much higher nitrate levels (say, 20 ppm), but run greater risks of the problems described above.
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thus 0.015ppm PO4 x 16 = 0.2 == 0.2 NO3 , again redfield.
Do you really think that if nitrate is 500 ppm, that the "optimal" phosphate level is several ppm?
Firstly, I've never used the word optimal. There are way too many variables to know that.
Ok, so lets overlook the obvious silliness of a tank having 500ppm NO3, let's say that is what the Original Posters reading was.
Lets pretend his PO4 reading was just 1ppm;
500 N : 1 P
In his tank with his mad growing chaeto in his sump (and we will have to assume unlimited Carbon and Iron etc) and no other nutrients added or nutrient consumers are present, his chaeto would grow and consume 16N : 1P ... up until the phosphate was depleted, leaving him with a steady 484 N : 0 P at which point the chaeto stops growing. That's not great for the tank and the OP would be having to dose P to allow N to drop more.
By having a tank with Nitrates and Phosphates AT the redfield ratio, thus 500 ppm NO3 : 31.25 ppm PO4 his chaeto would grow... and grow.... and grow..... and grow... and his nutrients would reduce and reduce .....484 : 3.025 ...... 468 : 29.25...... 452 : 28.25... etc until BOTH N and P hit zero together, which is much better for the tank and way more controllable for the reefer. He can also slow the rate of N and P dropping by removing chaeto, which would be essential when the levels of nutrients gets lower. No need to dose Vodka etc, no need to run GFO etc.
Personally, I would aim for just above the redfield ratio, say 18N:1P to ensure that the phosphate runs out first, leaving only a little Nitrate.
So the point is, is the ratio of nitrate to phosphate important, or is it the amount of each that is importnat?
That answer is easy, it's both! having an excess of anything doesn't do the tank any favours, it's wasteful, it's inefficient and it's that "out of balance" one way or the other why this forum and all the others have the same algae or cyano issue posts. Reefers dosing vodka to lower nitrates but leaving phosphates unchecked, wondering why they get a massive cyano outbreak, or reefers using too much GFO and stripping the phosphates out leaving the nitrates unchecked and wondering why they have fast growing algae issues.
All ULNS tanks run on the ability to keep nutrients low, typically 0.2 ppm NO3 : 0.015ppm PO4 as that's the smallest we can test.
I mean no disrespect in posing my arguments to you, but for you to say that there is no merit to a tank being run at/around redfield ratios for carbon, Nitrogen, Phosphorous is wrong.... it's what the typical seawater runs at, and what ULNS reefers use, both with great success.
factor varies a lot for different kinds of life. One of the lowest I have seen is 28:5.6 (
