How much sand to buy for a bed depth, and how much rock a tank of this size takes.
| Bed depth | Sand, lb | Sand, kg | Displaces |
|---|---|---|---|
| 0.5 in | 21 | 9.6 | 3.3 L |
| 1 in | 43 | 19.3 | 6.6 L |
| 2 in | 85 | 38.6 | 13.2 L |
| 4 in | 170 | 77.1 | 26.3 L |
Sand quantity comes from footprint times depth times bulk density, so it scales with the floor area and not with the tank rating. A shallow wide tank takes far more sand than a tall narrow one of the same gallonage.
sand = 5,574 cm² × 5.08 cm ÷ 1000 × 1.55 kg/L = 38.6 kg
Sand mass is footprint times depth times bulk density. CaribSea publish 85 lb per cubic foot for Special Grade, 90 for Fiji Pink, and 96 for the sugar sized oolites, which is 1.36 to 1.54 kg per litre. That is where the familiar figure of roughly one pound per gallon for a shallow bed comes from.
The mineral itself is denser than that. Aragonite grains are 2.93 g/cm³, so a settled bed is only 46 to 53 percent solid and the rest is pore space holding water. That is why a sand bed displaces about half what its bulk volume suggests.
Bed depth is a design decision with consequences. Under an inch is easy to keep clean and holds little biology. One to two inches is the common shallow bed. Four inches and deeper is a deep sand bed, which does real denitrification but goes wrong slowly and badly if disturbed after years.
The awkward middle, roughly two to three inches, is the one experienced reefers avoid. Deep enough to go anaerobic in patches, shallow enough to get stirred up.
Rock quantity per gallon is convention rather than chemistry. A pound per gallon was the old standard when rock was the filtration. Modern tanks with sumps and skimmers run less rock and more open water, and corals grow better with flow reaching everywhere.
Buy rock by look rather than by weight. Porous rock weighs half what dense rock does for the same visual bulk, so a pound per gallon of Pukani builds a much larger structure than a pound per gallon of man made rock.
Rinse dry sand thoroughly before it goes in, in batches, until the water runs clear. Skipping this means a week of cloudy water and a clogged skimmer.
Sources: CaribSea marine substrates · CaribSea FAQ, sand quantity · Hamza's Reef, Substrate Calculator · Webmineral, aragonite density