chemistry and implication of hardening of DSB

[yraveh]

My 2" live sand turned into a rock in many areas.
What's the chemistry behind it? do I loose any of the live sand metabolic/microbiological properties?

[Randy Holmes-Farley]

Here's a thread on it:

http://www.reefcentral.com/forums/showthread.php?s=&threadid=1521274

[Boomer]

The hardening of the sand bed can be two fold

1. It is called Early Maine Diagenesis of carbonate sand. If the pore water does not have good bioturbation or some type of pore water movement the pH drops and dissolves some of the Aragonite/Calcite sand. This then raise the pH, Alk and Ca++ back up to high levels, which then cause CaCO3 to precip out on the sand grains more readily and "glues" them together, as a precip of CaCO3 cement. So, it short, you have just made Limestone. This is also a precip of Hi-Mag- Calcite usually.

2. Bacteria. These guys often leach out an organic substance which can be quite sticky and clues the grains together. However, it is a rather soft "glue" so the grains pull apart easily. Where as in the above, you almost need a jack-hammer to get them apart. Again due to poor pore water circulation or poor bioturbation.

[mathias999us]

I had this happen too a while back. My clumping was more like the "needs a jackhammer" type that Boomer describes. But, interestingly enough, all the clumping occurred in front of my return jets, where the flow is highest. I will add that I top off with saturated kalk via ATO into the sump chamber, so right in front of the jets would be the first place fresh kalk ATO goes after a brief trip through the sump chamber.

Eventually I removed all of the clumped sections of sand, and I haven't had a problem since (almost 6 months ago since I did that).

[Boomer]

Mathias

Did you by any chance have more algae growing there also ? More algae will have a tendency to raise the local pH, which may do the same thing causing precip in that area.

[mathias999us]

Hmmm, interesting. Pretty much all of the sand clumps had the grains of sand turn dark green when the clumping occurred, which I presume was some form of microalgae. Without more data it's a bit of a chicken and egg situation. But your theory that "first came the microalgae on the sand grains, then came the clumping" is entirely plausible. So, I wonder what would cause microalgae to grown on the sand grains predominantly in front of the return jets (6 return jets, so it was a clear pattern for me).

[cs12109]

I had the same thing in my tank, " hardening of the substrate". I got 2 diamond gobies and now my sand is like new, white and no clumps!

[cs12109]

The cyano that has been plaquing my sand bed is also gone, gone, gone!