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Thread: Phosphate that won't go away

  1. #1
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    Phosphate that won't go away

    Some people are surprised when their nitrate goes to zero, but their phosphate only drops to a certain level, and stays there. Could it be that their scrubber "nitrate limited"?

    No. What is happening is that the rocks (and maybe the sand) are releasing phosphate back into the water, slowly. Rocks, and to a lesser extent sand, absorb phosphate like a sponge. When the scrubber removes phosphate from the water, the rocks put this phosphate back into the water. This can last for weeks to months, depending on how much phosphate was in the rocks, and how fast your scrubber is removing phosphate.

    At first, the phosphate coming out of the rocks can cause additional algae to grow on the rocks, but this will fade away after weeks or months. However, even when it fades away, phosphate is still coming out of the rocks. It has to, until the phosphate "inside" the rocks equals "zero", which is what the phosphate in the water is trying to reach too.

    The stronger your scrubber is, the faster this "zero" situation will happen. Also, the stronger your scrubber is, and the more flow-through it has, the more likely you will measure "zero" phosphate in the water even though some phosphate is still coming out of the rocks. This is because as soon as a little bit of phosphate comes out of the rocks and goes into the water, it is pushed into your scrubber and absorbed by the algae there before it can ever build up enough in the water to be measured. If your scrubber is weak, however, even a little phosphate coming out of the rocks will build up enough in the water to be measured.

    Ironically, the phosphate that you measure in your water can actually increase after the nuisance algae on the rocks goes away. Why? Because that nuisance algae was consuming the phosphate as it was coming out of the rocks; with no more algae on the rocks, the phosphate that comes out of the rocks just goes into the water, where it will be measured by your tests unless it gets sucked up immediately by your scrubber.

    Also, even more ironically, the more and faster your scrubber pulls phosphate out of the rocks, that more this phosphate will "hit" any corals that are on the rocks. And it will occur most at the bottom of the corals, where they are attached (and are nearest) to the rocks.

    So the normal progression of a tank with bad algae problems might be:

    1. Bad nuisance algae everywhere (water may test "zero" for nitrate and phosphate because the algae consumes it.)
    2. Add scrubber.
    3. Nuisance algae goes away from bare plastic, glass and sand, but increases on rocks. Nutrients in water may go up, or down.
    4. Corals may turn brown where they are attached to the rocks.
    5. Nitrate in water finally goes to "zero", but phosphate only drops to a certain level.
    6. Nuisance algae on rocks finally goes away. Phosphate in water may go up.
    7. Finally, all phosphate is removed; water tests "zero", and no more nuisance algae is in tank.
    Last edited by SantaMonica; 08-26-2012 at 10:52 AM.

  2. #2
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    Ok I'll bite. 120 gallon tank in the Dentist's office. I took over maintenance in December of 2009. Phos on API or Salifert was around 1.0. By the end of 12/09 it was 0.5-1.0. By mid-Feb, running RowaPHOS it was 0.03-0.1 on Salifert. Hovered between 0.03 and 0.15, maybe as high as 0.25 through until Oct 2010

    10/10/2010 the first scrubber was put on. Sized for tank, 4x T5HO lamps with TEK-II reflectors, grew algae like crazy. By Nov 2010 it was 0 (I have only one recorded test of zero between 10/10/10 and 5/2011). Replaced with second scrubber (same lights, better housing) in Jan 2011, grew 3D like CRAZY. By then P was 0.19, 0.11, 0.21 (Hanna meter now). In March 2011 I replaced the drain and return plumbing to increase flow. My test log shows it bouncing back an forth but generally between 0.09 and 0.16, which is no significant change from before.

    Tank crack developed and I moved everything May 15 2011. 4 days later, P 0.63 (no scrubber). Nitrates never went above 5.0 on Salifert. Scrubber installed after 1 week and by June 6th, at 0.14 and then continued to bounce around as before.

    Tank contents moved to new tank in Nov/Dec 2011. After full move and 1 week of scrubber running, P dropped to zero. From that point, it started to climb again and kept between 0.05 and 0.13 for over 6 months.

    I put the smaller LED scrubber on it at the end of June. Last week tested N=0 P=0. This week mid growth was N=0 P=0.03. Just took sample and will test again.

    But the long and short of it is that this system had an initial condition in Dec 2009, as best as I can tell, of P=1.0 or thereabout (N was also 160). This condition was addressed and N and P were reduced quickly (by Feb 2010) and maintained low for at least 6 months using PWCs and RowaPHOS and from LR and a skimmer & filter socks.

    Granted that from 10/10/10 until March 2011 the scrubber was probably under-flow. Then the top-of-tank scrubber was used for 6 months, then back to in-sump.

    That is a total of 20 months of powerful scrubbing in a tank that was not algae ridden to begin with, it was actually pretty clean looking. There was never any algae that bloomed from the rocks indicating leeching, there was not a significant clean-up crew, there were a few tangs but even those now don't eat much of the algae in the tank.

    So while I agree that your above scenario theoretically could be the case, I argue that it is most certainly not always the case as this it not the situation I have encountered, and it is not the situation others have encountered either.

    So at best, this is a theory based on a situation that may or may not be the case, and is surely not proven, either way. The bottom line is that many people experience this (N drop to zero and P climbs) and not all can be explained like this.

  3. #3
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    Last months experiment with a UAS... new Frag tank ... no sand... no rock.. few corals, fed 2x a week with reef chili. New water, tested 0/.01 before use, used screen from my ATS, after 1 week, nitrates still 0, phosphates went from .01 to .15. Explain that. (I know the answer, just seeing if SM can figure it out).

  4. #4
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    Glad your back Ace, although I have a feeling you never left. You just love it too much.

  5. #5
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    I love science and experimenting.. what I don't love is seeing all the terrible information SM is giving out. This post is a perfect example. He can't seem to admit the ATS/UAS has some limitations that require extra means to control. He has even gone so far to say he has always had 0/0 readings and then finally admits last week he never actually tested his phosphates, and when he finally does, they are .36!

    Quote Originally Posted by SantaMonica View Post
    I just used my Hanna digital meter that I bough last year for the first time... said 0.36. And the API said 0.25. N is zero on Salifert.

  6. #6

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    My tank is all the proof I need of the issue with nitrate limitations.... Nature has laws... we just need to know them... Photosynthesis is the most amazing "factory" ever created. However if one of the needed "raw materials (N)" are not available, when needed. In addition to having too much of another "raw material (P)", work in the factory slows or comes to a halt until enough material is obtained to complete, or the cell dies...

    SM, can you show proof otherwise?

  7. #7
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    Other way around...P available, N not...but works either way...

  8. #8

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    yep, noticed it soon as I hit post... lol its fixed

  9. #9

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    Well my case is the opposite of all of you, my tank is a year in December

    And before commenting serious errors no3 were always between 1.1 and 2.2 ppm (Lamotte) and phosphate between 0.02 and 0.06 ppm (hanna)

    After an accident in my aquarium, no3 jumped to a little more than 28ppm, but my phosphates rose slightly to 0.08 ppm

    After this event has been a battle no3 down, but otherwise my phosphates down to 0.00 and I have to raise them constantly, so that the algae can consume efficiently no3


    It's just my experience

    regards

  10. #10
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    I agree your tank is the odd exception tebo. How are you adding Phosphate to the system??

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