The Why’s and How’s of Substrates
Crushed Coral – Dolomite – Aragonite

For those of you who use a substrate in your tank with an under gravel filter, a plenum, or just as a layer on the bottom of the tank, you might want to know what’s in that substrate and why it’s there.

There a four reasons for “sand” in an aquarium.

1. Biology! Everybody whose been around aquariums knows that aerobic bacteria are necessary for ammonia control (translated – converts fish and animal waste to less harmful nitrates). A second anoxic (very low oxygen level) bacteria will grow given the correct conditions to convert nitrates into nitrogen gas that will bubble away.

2. pH control! When ammonia (NH3)breaks down using oxygen, three “acid” ions are formed. If it weren’t for some form of calcium carbonate (CaCO3), your tank would be as acidic as vinegar because crushed coral, aragonite and live rock “eat up” acids. Every wonder what pH stands for? It Latin it’s pondus hydronium – in English it’s positive hydrogen. The chemical symbol (H+1) represents acid. For those of you who want to know the math. Call me if you’re interested and I’ll explain.

3. Calcium! Calcium (Ca+2) is released when those “acid ions” from ammonia react with Calcium Carbonate.

4. Looks or esthetic value! You have choices in size of crushed coral from fine #0 sand to coarse #5 gravel and specialty substrates like puka (a nice mix from #1 to #5 crushed and broken seashells).
The old stand by is crushed coral. Size varies from #0 (very fine sand) to #1 (more coarse than regular sand) to #5 most commonly used then up to #10 (pea to kidney bean size). Composition depends on Mother Nature and the animals in a particular part of the ocean where it was created, but it is basically Calcium Carbonate . An assortment of metals are trapped in the rock besides Calcium. Most commonly, Magnesium, Strontium, and Molybdenum which are regularly supplemented.

Dolomite is a 1:1 mixture of Calcium:Magnesium Carbonate (CaMgCO3). Nature forms a very large crystal that resembles a saddle which for aquarium purposes has its draw backs. Large crystals do not react with “acid” quickly, therefore pH control is poor because the crystal is slow to dissolve. As a result, calcium stored in the crystal structure is not easily released.

Aragonite minerals are carbonate minerals that have a formula of ACO3 where A can be either Calcium (Ca), Barium (Ba), Lead (Pb) and/or Strontium (Sr) but a dissimilar structure to dolomite that allows it to react easily in an aquarium setting. Don’t panic because Lead is present. It is 100,000 more difficult to dissolve than the Calcium, Strontium and Barium. The Lead remains as an insoluble, harmless dust. Barium isn’t harmful in the quantities present and it is added during the manufacture of salt mixes. Because it is a soft, white mineral, it reflects light well and it is great for wrasses, gobys, sand stars, tube worms. This is what we recommend for all saltwater aquariums.

Be aware that the Southdown play sand #00 is cheap but it is so fine that it takes days to settle and it blows around the tank very easily. Many people wish they had never used it. If you use a plenum with it, It filters through the screen and destroys the plenum. If you use a deep sand bed, the sand packs so tightly, the lower half goes anaerobic, turns black and smells like rotten eggs.  Aragonite less than 1 inch deep avoids all those problems.  Otherwise a plenum avoids anaerobic problems.

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