McPhock;943578 wrote: Here is specifically what I was referencing:
http://www.wetwebmedia.com/deepsandbeds.htm">http://www.wetwebmedia.com/deepsandbeds.htm</a>
"Compositionally, any substrate you choose is almost certain to be made of aragonite or calcite. Both are forms of calcium carbonate, but arguably aragonite is to be favored for it's better solubility and buffering capacity in seawater (providing necessary bio-minerals, buffering pH, and supporting calcification). Aragonite can begin to dissolve, in fact, at a high pH over 8.0 (a still safe level for marine life), while calcite does not readily dissolve until the pH falls well below 8.0. This means that calcite is not likely to impart any significant benefits (buffers/alkalinity) into the water until the pH falls to a level that is too dangerous for most marine life. In this regard, the old argument of dolomite & crushed coral versus non-calcareous freshwater "gravel" for marine aquariums in the early days was a moot point (they were all calcite). "
Again though, I have no personal experience in this matter.[/QUOTE]
My source is Randy Holmes Farley, but I do respect Bob Fenner very much as well. Per Randy, if you have a stagnant, anaerobic zone in your substrate, you could have aragonite dissolving if the pH were way below that of the water column. But as the goal of most reefkeepers with a substrate is to have it regularly stirred either manually or by crittiers, keeping it aerobic, then that is usually not a concern.
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[B]From Reefs.org:
Dissolving aragonite utilizing sand-bed gradients[/B]
Randy, from reading your articles I have gathered it is relatively hard to use an aragonite sand bed as calcium/alkalinity support in marine aquaria--due to the factors of saturation, pH etc as preventative mechanisms...
My question is, does the amount of carbonic acid generated by aerobic bacteria metabolism in a normal sand bed create enough of an acidic region to break down aragonite substrate? although its probably not enough to provide the sole alkalinity/Ca+ support needed, it is interesting if there is degradation of the substrate because of this occurence. It is also interesting if there is enough degradation of the substrate that would require regular additions to your sand bed to keep it from dissolving away eventually...
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[B]Randy's response:[/B]
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by Randy Holmes-Farley
Aragonite will not dissolve at any appreciable rate if the pH is not reduced below that of the tank itself, because the water is always supersaturated with respect to calcium carbonate.
If the pH is lowered for any reason inside a sand bed, then the calcium carbonate can begin to dissolve. The pH down inside my sand bed is substantially below that of the water above the sand (mid to upper 7's compared to 8.4 above it).
The pH in the sand can be lower do to either aerobic or anaerobic respiration where organic materials are broken down. Both produce about the same net amounts of acid.
That said, if the sand is aerobic and organics are breaking down, that necessarily means that there is significant O2 exchange between the water column and the sand.
If there is substantial O2 exchange, then there is likely to be a lot of H+ and OH- exchange (and other acids and bases), and that will limit the pH drop that is experienced in the sand.
So in that sense, low O2 regions may be more likely to experience sand dissolution, but that has little to do with the O2 itself.
One problem is that even if the pH drops low enough for some CaCO3 to dissolve, the calcium and carbonate/bicarbonate are going to be slow to get out, for the same reasons that the acids and bases in the water column are slow to get in.
So it is not an especially efficient process for dissolving sand. It is also dependent on the degradation of organics in the sand, which is likely to be dependent on the nature of the organsims living and moving about in the sand.
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Link to the exchange: [IMG]http://www.reefs.org/forums/topic25814.html">http://www.reefs.org/forums/topic25814.html</a>
