Ulns has been used so much I dont know where to begin with what it "really" is. However for the sake of this article it means that you have a no3 and po4 deficit.
What is a no3 and po4 deficit you ask? It means that if you were to dose phosphorus and nitrogen in to the system that it would eat it up and get you down to 0 ppm without any water changes.
Contrary to popular belief. A REAL 0 ppm no3 and po4 reading is a major problem. You will witness corals pale, turn bone white, while people tell you to feed more or turn down the lights. There is some truth to this but let us get to the heart of the matter shall we?
A long time ago some old guy (who is gone now) discovered something called the redfield ratio. (google this for more info as i will do a synopsis)
Redfield ratio and all it's spin-off theories state that photosynthesis occurs in the ocean at a near universal rate of 106C:16:no3:1 po4.
Now while I wont get in to the deviations of benthic macroalgae or of other life. It is a REAL GOOD IDEA to use it as a guideline.
So what does that mean to the aquarist?
Well it means if you do not have no3, or po4, or carbon.... bye bye corals. They have nothing to photosynthesize. They cant use alternative fuel like lithium or plutonium to synthesize... so they pale. The zooxanthellae which is a dinoflagellate is the same as algae. (in layman's terms). They need it. And you probably have just enough no3 and po4 to get them hold on for dear life.
How do we solve it?
Well we know that po4 is the LEAST needed for photosynthesis. So getting it is easy to come by. Even by my tank which sucks up nutrients like a sponge. Every time you feed, touch, or even look at the tank, po4 will be in the water column. The only exception to this is a GFO overdose. If you are overdosing GFO you will basically have nothing for it to photosynthesize. Remember the redfield ratio of 16no3 to 1po4? yeah... it's a marriage. Need em all buddy. So make sure you have some.
I have observed that po4 measurements of .02ish is optimum. However up to .08 is fine as well. (although some life will get irked)
no3 should be around 2ppm - 5ppm for optimal coral coloration. If 2ppm or below is observed for longer than a week, corals will pale. Mine will pale within 72 hours. It is essential to keep no3 above po4 to prevent nuisance algae like GHA and to not get in to the cyano trap of po4 being greater than no3.
How do we avoid this? Well we can use sodium nitrate (the byproduct is salt and relatively safe) or potassium nitrate (byproduct can be potassium and possibly overdose). I choose the latter because I do have a potassium kit and it is readily available within Seachem's Flourish Nitrogen formula. Care should be taken and warning should be given. I have not observed any detrimental effects, however if you want to go the powdered "pure" route, you can look up salt peter or spectracide stump remover from lowes which is essentially the same stuff.
Alkalinity has been also an issue with ULNS. I have observed alk above 8 to make my birdnest recede and even RTN if approaching the 9s. Many a birdnest I have lost wondering what my issue was until I dropped alk down to the 7s. I personally maintain alk between 7.2 and 7.8 at all times. At 8 or above... my sps get irritable. This could also be the issue with ulns and high alk where burnt tips occur due to the skeletal formation of calcium carbonate forming faster than the photosynthetic ability of the coral via the zooxanthellae.
LIGHT:
If you have strong strong light and you have no nutrients, you are really just stressing the heck out of the sps because there is nothing to photosynthesize. You need these nutrients, and the less you have, the lesser your light should be discourage photosynthesis. We are always told more light, more par, more pur, but in the end, if the building blocks are not there... degradation occurs. There is no reason to have a 1000 hp engine, if you have no gas to go somewhere.
