Calcified Sand or Bacteria Secreting Glycocalyx?

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Steve2020

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Tank has been running for 2yrs. I have had some hard sand in some small areas for about 14mo and in the last 3-4mos it has gotten worse. Sand bed is shallow ( 1-1.5" ). I had Dino's for about 3mo and they have been completely eradicated for the last month or so. My eradication consisted of adding more pods, dosing phyto, MB7 and MB clean alternating between the two and adding silicate to induce a Diatom bloom. I also allowed my nutrients to increase to NO3 of 19ppm and PO4 to 0.26ppm. Tank is Dino and Diatom free so I decided to remove a portion of the hard sand. The green substance on the sand is not slimy and will not rub off. I stirred up some sand in some saltwater and could not find any organism under a scope. It is actually hard and looks like coralline algae. I put the hard sand in a container and added some hot water with some vinegar ( 70/30 ratio ) and it started to bubble up instantly and made a fizzing sound.
My tank nutrients and chemistry as of today: NO3=7.9ppm
PO4=0.12ppm ( still high but coming down slowly )
Alk=9.1
Cal=450
Mag=1510
ALK, Cal and Mag have been consistent since day one of tank setup. I match my salt mix parameters. ALK and Cal are dosed in equal amounts 8 times a day and dosed 1/2hr apart from each other with CAL being dosed first.
So, is this Calcified Sand or maybe bacteria secreting Glycocalyx?
Why did the sand bubble up when vinegar was added?
What could the green substance on the sand be besides Coralline Algae? Again, green on the sand is not slimy and will not rub off. It is stuck to the hard sand.

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Steve2020

Steve2020

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i always thought coralline is red
I just copied and pasted this from an article: "The colors of these algae are most typically pink, or some other shade of red, but some species can be purple, yellow, blue, white, or gray-green."
I have purple and green with the occasional bright pink. The bright pink shows up in dime size spots and then changes to purple or disappears. Not sure why.
 
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JNalley

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Vinegar, Hydrochloric acid, etc react to calcium carbonate (Calcite), so it's not surprising that you're getting that reaction. Are you dosing anything for your Alkalinity? Like Sodium Bicarbonate or Sodium Carbonate? That can cause cemented sand from precipitation. Generally, if it's from bacterial secretions, you can easily collapse the matrix back into fine-grain sand.
 
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Steve2020

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Vinegar, Hydrochloric acid, etc react to calcium carbonate (Calcite), so it's not surprising that you're getting that reaction. Are you dosing anything for your Alkalinity? Like Sodium Bicarbonate or Sodium Carbonate? That can cause cemented sand from precipitation. Generally, if it's from bacterial secretions, you can easily collapse the matrix back into fine-grain sand.
I am dosing BRS 2 Part. My salt is HW Reefer Marine Mix which I measured at 9.1ppm DKH, 452ppm Calcium and 1510ppm Mag. I auto dose Alk and Cal 8 times a day to maintain the salt mix measurements and dose Mag when it drops to 1450ppm which is about every 4-5mo. My Alk, Cal and MAG have been stable at these values ever since I started my tank 2yrs ago. I removed the calcified sand and added some dry sand yesterday and today, and below is what it looks like today. Time to add some more corals.
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