By Bob Goemans
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Bob Goemans corresponds with Sergio Angular

Greetings Bob,

Hope all is well. I am having a problem with my CaribSea crushed coral. Its 2 years and 8 months old and in a 20 gallon tank, with a 10 gallon bare bottom sump. This is a mini reef with 40 pounds of special grade reef live crushed coral. It also contains about 20 pounds of live rock. In the tank I have 1 damselfish, 1 emerald crab, and a couple of snails. My corals are star polyps, a colony of pumping Xenia that are splitting and mushrooms that are growing all over the tank, but recently are fading in color. The equipment I'm using is a CPR skimmer box, a Marineland 360 canister filter with ROWAphos in the filter, and a RIO 1400 pump in the sump as a return. I also just purchased a Tunze Nano protein skimmer (still breaking in). The lighting is a Coralife compact with one 65 watt actinic and one 65 watt 10K lamp. I have been using a B-ionic 2 part calcium and alkalinity product and also their (ESV) magnesium supplement and iodide supplement. Water changes are about 4 to 5 gallons a month. My readings are; ammonia less than .25 ppm, nitrite 0, nitrate1.0 ppm, salinity1.028, pH 8.0, temp. 79F, phosphate 0, alkalinity 9.9, magnesium 1380 ppm, calcium 380 ppm and iodide undetectable. All tests are with Salifert kits.

My problem is that the substrate has hardened solid! I have about a 1/2 inch of loose crushed coral on top, but underneath it is hard as rock. I believe it to be some form of precipitation, if this is correct what can be done to rectify this? And will it be safe to break it up without releasing anything into the tank? The small piece I overturned in the corner was visibly clean with no detritus or discoloration, and the depth of the substrate is 2 inches. I feel that most of this should have been avoided by better awareness to the substrate as I did vacuum only the top layer. Maybe I should have stirred up the bottom also. I am still traveling on the LIRR while enjoying and learning from your articles in FAMA.

Thank you for your time.

Sergio Angular

Seaford, LI, New York

Hi Sergio,

Thanks for your email. The Long Island Rail Road! Been many moons since I've lived on Long Island!

Past research has shown that substrate hardening is caused by either of two situations, and its quite possible both can happen in the same sandbed. One finding is that hardening is caused by fast forming colonies of bacterium that rapidly change the size of the colonies to stay in balance with the level of 'food' coming their way so to speak. There is some thought these bacteria secret compounds outside their cells that cement grains of sand together forming chunks of sand particles. Generally, if formed by bacteria the chunks can easily be broken up with your fingers and returned to loose sand particles.

The other reason for binding is related to precipitation 'within' the bed. As we know, the purpose of using good grade aragonite sand, such as the CraibSea brand, is that this very soft type of calcium is easily dissolved and therefore somewhat supplements the calcium level in the bulk water. However, if the bulk water's calcium content is above that found in natural seawater, in other words out of balance with its counterparts that makeup alkalinity, it cannot enter the bulk water and therefore precipitates as calcium carbonate upon adjacent sand particles, securing locking them together. If resulting from this type precipitation, these chunks are more difficult to breakup.

I've personally seen both easy and hard to break up sand chunks in some past aquariums. Past experience seems to indicate that precipitation was the cause in those that experienced hard binding, as those systems all experienced high bulk water calcium levels in a hope to increase stony coral growth. In systems where 'alkalinity' was maintained in a balanced way with what appears in the wild, no binding was incurred.

Nevertheless, all sandbeds should be vacuumed thoroughly right down to its bottom once a month. Any damage to bacterium or shifting of different classes of bacteria to different substrate depths will quickly be restored naturally as bacteria are quick to reestablish themselves. Cleanings such as this will help prevent binding or at least locate areas that may begin to bind. If any binding is found, your fingers should be able to crumble them so as to restore the chemical pathways, which appear to be in your letter impaired, as your ammonia level is far too high. That indicates the areas for substrate bacterium are not sufficient, as the depth of the bed has hardened, leaving not enough sand depth to house the proper types of bacteria! (Breaking up binding substrate will not harm anything in the aquarium, in fact, just the opposite, as there will be more area for bacteria to colonize and do their job!) Either break up these hard areas or if not possible, remove them and replace with new substrate. Also your salinity/specific gravity is far too high. Drop to 1.025.

One item further is the looks of your mushrooms. I've found that once nitrate drops below 3 mg/l, mushrooms begin to fade out and dwindle in size. I've tested this, and found once nitrates rise above this level, mushrooms begin to take on a more robust size and coloring. Of course, keep in mind they prefer slow water movement and medium to low light areas.

Hope this helps,



Sandbed Hardening

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