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Aquarium.Net April 97

Sam continues his series on Sand beds, Aquarium Net has numerous articles written by the leading authors for the advanced aquarist

Sand Clumping

By Sam Gamble

In spite of our best efforts, we haven't achieved the "in situ", all natural aquarium. We're doing better all the time. Communication is a very helpful part of it. Thanks to people like D. R. Martin (Aquarium.Net), communication on the INTERNET is helping many. Problems and observations, as well as, theory and solutions are quickly exchanged on many tropics.

One such topic is "clumping' of sand substrate. The clumps or coatings are actually calcite deposits caused by precipitation. The observation is new. New in the sense it has become noticed with the use of sand substrates for passive filtration. In other words since going beyond under gravel filters and the Berlin Method. This is important to consider because each time new methods of aquarium filtration are implemented, there is the tendency to carry over some technical aspects. Some bring new puzzling observations.

This is the case of sand bed filtration and clumping. Several factors from previous methods contribute significantly. Kalkwasser and buffering are two. The contributing ingredients are calcium compounds, and particularly the Ca ++ ion. In natural seawater calcium carbonate (CaCO 3 ) is supersaturated and the important element being calcium (Ca ++ ). Since our closed system aquariums are finite and don't have an infinite supply of important constituents like calcium, we add them.

The second factor that influences calcite precipitation ("clumping") is less obvious, but important to consider in closed systems. Magnesium inhibits the chemical precipitation of calcite under natural conditions. So then we essentially have two sides of the coin; adding too much and removing too much. A reoccurring situation in closed system aquariums. A familiar battle between maintenance of a finite system (the aquarium) versus the observations in a relatively infinite system (the sea).

These are the predominant pathways that Richard Greenfield, Marine Geologist of CaribSea gave in a personal communication. However, as he says, it is a bit too complicated to issue a general statement. You can say it is a bit unsightly, but doesn't harm anything. Many find it easily controlled by sand sifting organisms.

For the sake of understanding, lets take a look at the pathways that cause calcite to precipitate from solution, forming crystals. Thereby, perhaps preventing it in the first place. The first path is simple supersaturation in respect to CaCO 3 through the audition of Kalkwasser solution (Ca(OH) 2 ), calcium chloride (CaCl 2 ), and buffer (in the form of carbonate (CO 3 ) or various combinations of these chemicals.

Sea water is supersaturated with respect to Ca to begin with. One can increase supersaturation easily by increasing the ionic strength of these ions. It can be done by direct addition or even evaporation. Another way could be done is by raising the pH with repeated water exchanges.

Kalkwasser, however, seems to be the common denominator. This would mean that precipitation is often caused by increasing the degree of supersaturation and increasing the pH (keep your eye on the OH - ion). It is important since the distribution of Ca ++ ions by Kalkwasser is probably not as active as other inputs such as aragonite or CaCl 2 . Importantly, a person can get precipitation of calcite even at low Ca ++ and carbonate concentrations. It occurs in a system that has naturally high pH levels (8.3 and above).

When a carbonate precipitates, it prefers to do so on other carbonate materials such as aragonite reef sand. Without sand in the system you would expect calcium carbonate to come out of solution in the more energetic areas of the system such as the pump and plumbing. This was an often observed event before the use of a sand substrate. Pumps and plumbing needed periodic cleaning to remove scale.

The easy solution to most "clumping' is to reduce Kalkwasser input. A word of caution for changing regular protocol; if you decide to reduce the input of Kalkwasser, do so gradually. The presence of the hydroxyl (OH-) ion may play a significant part in the respiration of hard corals particularly in aquariums that tend to accumulate dissolved carbonate or have rather sluggish circulation or both.

The second pathway is more insidious in that it involves the subtraction from solution of a rarely monitored ion, namely Mg ++ . Magnesium is roughly three times more abundant than calcium in natural sea water (1.295g/kg versus 0.412g/kg). Yet it plays a vital part in the whole carbonate scheme of things. This is what keeps the oceans supersaturated in respect to calcite.

Magnesium poisoning (a chemical term that has no implications to living organisms) inhibits the chemical precipitation of calcite at normal ocean values of pH and ionic strength. The process is by coating the surfaces of growing calcite crystals with very unstable combination of Ca and Mg carbonates and hydroxides. If magnesium were to be subtracted from a closed system in significant amounts you could expect to see accelerated calcite crystal growth. It could be observed even at normal pH, Ca, and carbonate ionic strength. So how could you subtract this seemingly spectator ion out of a closed system?

There are two ways of doing this. First, every reef keeper's favorite algae is the pink coralline variety which is depositing high magnesium calcite (geologists call it high mag calcite). Sometimes it grows at a spectacular rate. This calcite can be 30 mole % magnesium. Also, the calcite crystals growing on aragonite substrate can contain a similar percentage. Since calcium, not magnesium is the only +2 ion regularly replaced outside of water changes, you may deduce that many closed systems can experience a steady decline in dissolved magnesium. Hence a commensurate tendency to chemically deposit calcite results. This is a good time to remind ourselves, that aragonite does not incorporate magnesium. Hard coral and Halimeda growth should not affect dissolved magnesium levels.

The best way to insure enough magnesium in a system is probably through regular water changes. However, as marine system technology and practice improves, water changes tend to become less frequent. Magnesium additions can be made from magnesium chloride or hydrated magnesium sulfate (Epson salts). I don't know of any commercially available test kits for magnesium. So if readers do want to make magnesium additions, do so carefully and at your own risk.

Again, "clumping" does not present any threat to aquatic life (including denitrifying bacteria) nor affect the performance of aragonite substrate outside of some reduction in porewater space. The calcite crystals ("clumping") themselves are metastable at whatever pH they were formed. This makes it likely you may even expect some buffering capability at elevated pH levels.

Also, assuming that these two major mechanisms postulated here are essentially correct, the conditions that cause calcite crystals to grow in the substrate have nothing to do with the substrate itself. In the past (before substrates) "clumping" would appear in the form of "chalky deposits" on and in aquarium fittings. Basically, when water conditions favor calcite deposition, it is going to come out of solution somewhere. The substrate is just the first choice.

When you see the calcite crystal formation on the substrate, you are probably seeing an overdose of Ca ++ . Since closed systems or particularly reef tanks, are decreasing the need for frequent water change maintenance, the increased Ca ++ could be coupled with a Mg ++ deficiency. It does contradict the notion that supersaturation of calcium is the best thing for coralline algae growth. How many times have we felt that when the coralline growth is below par, that the increase in alkalinity is will help?

The understanding that clumping is a result of several concomitant events and pathways is useful. It deters from the reasoning that perhaps another grain size of calcium based sand will change the mechanism. Secondly, it helps the understanding of mineral deposition by coralline algae. Most importantly, it's good to know that there isn't any particular danger for the closed system in which it happens. It is a little unnatural looking though. Who has ever seen in the reef environment, what looks to be melted ice cream? Then again, what reef develops input of calcium beyond supersaturation to the point of precipitation?

Questions and comments Sam Gamble 102170,3150 compuserve.com

Created by liquid
Aquarium.Net
Last modified 2006-11-18 19:42
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