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Originally Posted by Tangster That's just about the whole story in a DSB all the acids build up as the waste sulfides are very acidic and when heats up and migrates to the higher better oxygenated areas of the sand where it then it comes into contact with the more alkaline harder carbonate waters above then it will crust over and fuse just as concrete does and then it's pretty much the first indicator the whole thing is about to head south really fast.. Its been the major limitations of a DSB for 40 yrs.. Herbert Axlerodd and Spotte wrote about it first in 1970 was the first I read a bout it when we all where looking for the perfect natural system.. |
Chemical Solidification
Many chemical reactions don't happen like the reef literature says it does once the sandbed is established and the sand is coated with bacteria and organics. For instance, everyone told you that you need to use aragonite substrates because it will dissolve in lower pH situations and buffer your water. As soon as the substrate becomes coated with bacterial biofilms, almost all of the chemical reactions that occur are controlled by bacteria. Actually, that's a good thing....otherwise you would need to add more sand to your system every couple of months.
However, on newer sandbeds that are not fully populated with bacteria, organics, etc., these chemical reactions can occur and cause solidification.
While sulphide reactions can cause this to a certain degree, this really only occurs in a completely overwhelmed DSB (too much organics, not enough sand stirers, not enough Oxygenation, etc.) or a fairly new sandbed. For instance, Hydrogen sulphide zones are formed and then removed on a regular basis. The by-product of anaerobic Sulphate Reducing Bacteria (SRB's) is our nemesis Hydrogen sulphide. SRB's are limited by the amount of organics in the DSB as there will always be plenty of sulphate. (In other words, if you overload your DSB with organics, these guys can reproduce and produce a lot of H2S). In a system that is not overwhelmed and has plenty of bioturbators, the Hydrogen sulphide is harlmlessly converted by aerobic SOB's (Sulphide Oxidizing Bacteria) before it can even become a problem. The chemical reactions don't usually occur in an established sandbed because bacteria don't allow them to occur.
There's a number of things that can cause this but the cause of most solidification of a DSB is an accidental overdose of Kalkwasser or buffers and too low of a level of Magnesium. This usually is from people trying to push their Kalk or two-parters too much. This causes the alkalinity to go up too high causing Calcium to fall out of solution. The resulting calcite (Calcium carbonate) that forms locks the sandbed particles together like a brick. Obviously, this is only occuring at the surface of the sandbed. Depending on the size of the overdose (and resultant precipitation), this might just make some crystals stick together. If you have a large precipitation, then you can create a crust on the surface of the sand thick enough that bioturbators (sand-stirrers) cannot break through.
This can set up a chain reaction that can turn your entire DSB into a brick and it is hard to to stop it once it starts. (I.e. Oxygenated water cannot get into the sandbed anymore, bacteria continue with their respiration, all of the Oxygen gets used up, CO2 increases, bacteria (and their biofilms) die en masse, pH drops, some chemical reactions do take place causing more fusing, ......end result is a sandbed brick.
If you keep a DSB, it is important for you to have an battery backup airpump. Bacteria don't stop respiration just because the power is out. You should also have something to keep the water moving over the sandbed (even if it's nothing beyond your hand moving water). A long power outage can cause dissolved Oxygen to drop, CO2 to increase, bacteria to die.....sandbed brick.
Biological Solidification
Some bacteria create a substance called glycocalyx which is like a glue to sand particles.
glycocalyx sediments - Google Search
Additionally, some worms can cause problems.
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Phyllochaetopterus is generally a benign member of the detritivore group found in reef tanks and they are pretty good scavengers; any particulate material that strikes their tentacles is pulled into the tube and eaten. However, they are gregarious and reproduce well in reef tanks. These two properties may, in time, cause some problems. The worms can form quite large mats with literally hundreds to thousands of tubes all cemented together. These mats quite efficiently exclude other worms from the area, and can seal off the sand bed surface. This, in turn, can cause the emigration of other animals out from under such a patch, which can result in the cementation of sediments under the patch and the failure of the sand bed biological filtration under the patch of worms. Such worm masses need to be periodically broken up or removed from the system, otherwise they may cause the complete failure of a deep sand bed. The reproduction and subsequent patch growth can occur with surprising speed. In a 60 gallon hex tank I once had, I introduced some of these worms and, within about three months, they had literally paved the sand bed surface with their tubes. I attempted to remove them all, and it was not an easy task.
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The Worms Crawl In… by Ronald L. Shimek, Ph.D. - Reefkeeping.com
OK, how do you stop this from happening?
- First, never have a newer sandbed.
Ok, since this is not really possible, be aware that overdoses of Alkalinity/Calcium with high pH and low Magnesium can cause some major problems for you. (Test your Magnesium level). - If you use Kalk, don't drip faster than you should to get more Calcium. It's a reciped for disaster. (Besides, corals only need 380ppt...trying to go to 500 will just cause precipitation.)
- Prevent organics from building up in the sandbed. This means you shouldn't cure LR over a brand new sandbed, should have good flow in your tank to prevent food and detritus from settling in the bed, don't overstock your tank, don't overfeed your tank.
- Have plenty of sandstirrers to continually bring Oxygenated water into the aerobic zone, break down excess food, etc.
- Watch for excess growth of the worms I showed above.
- Periodically, use a spoon and check for sandbed chunks in the UPPER layer of sand.
- Raise your LR on pylons above the sandbed to prevent anaerobic zones.
