Saltwater Aquarium Sand – How Much Do You Need?


When dreaming about having a saltwater aquarium it is only natural that the focus is on the fish, the colorful invertebrates, and even the rock formation. Something as mundane and boring as the sand or substrate doesn’t get much thought until you have to buy it.

At which point the question will come up: ‘How much sand will I need for my saltwater aquarium?

Roughly 2″ of substrate is recommended to cover the bottom of saltwater aquariums with a shallow sandbed, and 6″- 8″ of sand for deep sandbeds. A general rule of thumb is around 1 – 1.5lbs of substrate for every gallon in shallow sandbed tanks, and 3 – 4lbs per gallon for deep sand beds.

In my opinion, sand, or substrate as it’s commonly referred to, helps to create a saltwater aquarium with a very natural look, but it does need work to keep it clean and maintained. Many aquarists run their aquariums with no sand, also known as ‘Bare-Bottom’, with great success.

Sand, or no sand, is purely an aesthetic choice you have to make when installing your aquarium, and having no sand will not harm the inhabitants if that is the route you wish to take.

Let’s take look at what is required if you decide sand is the way to go in your new reef…


To help you reference what we are talking about in this article you can find a great selection of both Dry Substrates and Live Substrates here at Marine Depot.

TBR Recommends

CaribSea Arag-Alive Fiji Pink Sand

Live & Dry Reef Tank Substrates


What are Deep Sandbeds Vs Shallow Sandbeds in a Reef Tank?

Before you can begin to calculate how much sand or substrate is going to be needed for your new aquarium, you have to work out which kind of sandbed you are going to install. The difference between the two types will dramatically alter how much sand you will need to acquire.

Richards Reef Dry Rock
My Reef Runs A Shallow Bed

A shallow sand bed is the most common depth of sand used in today’s aquariums. A 2″ layer will ensure there is enough sand to make your aquarium look natural and it is easy to keep clean, however, it will provide very minimal biological filtration.

A deep sandbed between 6″ – 8″ in depth will create an oxygen-DEPRIVED zone in the lower levels of the sand. This area is known as an Anaerobic Zone and will contain bacteria that help to consume nitrates and process them into less harmful nitrogen gas.

The main problem with deep sandbeds is that if these lower levels ever get disturbed, say by accidentally sticking a substrate vacuum too deep into the sand, deadly hydrogen sulfide gas can be released into the tank and kill everything. The hydrogen sulfide is created when pockets of detritus become trapped and begin to decay.

With the increased efficiency in today’s modern filtration equipment, the benefit of having the deep sandbed is not worth the risk of an accidental disturbance and ensuing tank crash.

For minimal biological filtration, shallow sandbeds form an Aerobic Zone for the growth of beneficial bacteria to break down waste into nitrates. This is an oxygen-RICH environment and does not pose a threat when the sand is moved around during cleaning.

This is the main reason why you will see most reef tanks with a 2″ to 3″ sandbed.

Reef Tank Sandbed Grain Size

The second part of calculating how much sand you need is to figure out what size sand grain you wish to use. With varying grain sizes, the volume of sand you will need to purchase will also vary.

Lets take a look at the types of grain size you can buy:

Fine & Medium Grain Sand

From the point of view of aesthetics, the fine and medium grain sands give a striking impression with their smooth, natural texture, but if the depth of sand is less than two inches the finer grain sands are light enough to be blown about by strong wavemakers and powerheads.

Too thin a layer of sand will risk exposing the bottom of the tank. It will look horrible and annoy the heck out of you, trust me, I’ve been there!!

Fine Grain Aragonite Sand

When using finer-grain sands you will have to play around with the positioning of your pumps to ensure the sand grains do not get blown around. This is easily accomplished, but will take a little ‘finessing’.

Here are the sand quantities for selection of tank sizes using a fine/medium grain sand – Prices Based on 10lb & 20lb bags of CaribSea Arag-Alive Fiji Pink:

Tank SizeQty for 2″ DepthPrice for 2″ DepthQty for 6″ DepthPrice for 6″ Depth
20 Gallon
30″L x 12″W
37 lbs$90112 lbs$275
30 Gallon
36″L x 12″W
45 lbs$120135 lbs$320
55 Gallon
48″L x 13″W
65 lbs$165195 lbs$460
75 Gallon
48″L x 18″W
94 lbs$230270 lbs$625
100 Gallon
72″L x 18″W
140 lbs$320405 lbs$950
180 Gallon
72″L x 24″W
185 lbs$430540 lbs$1,240

If your aquarium dimensions are different, please refer to this awesome sandbed calculator from Marine Depot:

If you are unsure of the density value of your chosen sand, you can find out that value HERE at the CaribSea website.

Coarse Grain Sand

If you are partial to the aesthetic appeal of coarse sand you may also use it on both shallow and deep sandbeds. Keep in mind though, that this kind of sand will tend to trap waste between the sand grains and will require regular vacuuming to keep it clean.

If a deep sandbed is the way you wish to go, the recommended way to create that bed is to use 5″ of fine/medium grain sand with a 1″ top layer of coarse grain sand.

Crushed Coral is The Largest Grain Size In The Hobby

By doing this it is easy to vacuum the top 1-2″ of sand layer to remove all the detritus and to keep the lower 4″ of sand undisturbed.

Coarse grain sand is also an option if you have very high-flow pumps since coarse sand is heavier than fine sand and is less likely to be blown around. Because the grains tend to blow around less, you can also reduce the depth of your sand bed from two inches to 1.5 inches, if you wish.

Here are the sand quantities for a selection of tank sizes using coarse grain sand.
* 2″ Sandbed Prices Based on 15lb & 40lb bags of CaribSea Florida Crushed Coral (72lbs/ft3)x
* 6″ Deep Sandbed Qty & Prices Based on 5″ of CaribSea Arag-Alive Fiji Pink & 1″ of CaribSea Florida Crushed Coral

Tank SizeQty for 2″ DepthPrice for 2″ DepthQty for 6″ DepthPrice for 6″ Depth
20 Gallon
30″L x 12″W
30 lbs$5095 lbs – F
15 lbs – C
$260
30 Gallon
36″L x 12″W
35 lbs$50110 lbs – F
20 lbs – C
$325
55 Gallon
48″L x 13″W
50 lbs$75165 lbs – F
15 lbs – C
$450
75 Gallon
48″L x 18″W
73 lbs$100225 lbs – F
35 lbs – C
$585
100 Gallon
72″L x 18″W
110 lbs$150340 lbs – F
55 lbs – C
$860
180 Gallon
72″L x 24″W
145 lbs$200450 lbs – F
70 lbs – C
$1,050

Again, if your aquarium dimensions are different, please refer to the sandbed calculator from Marine Depot:

What Kind of Sand is Best to Use in Reef Tanks?

The most commonly-used sand in saltwater aquariums is aragonite sand, which is essentially calcium carbonate. Aragonite sand is made of pulverized coral skeletons and mollusk shells and the main reason for using aragonite sand is that it has low levels of silica.

High silica-based sands can cause diatom problems which are seen as a form of brown algae dusting over the sand. Every saltwater aquarium will get this algae during its first few months, and with patience, it will pass as the silica becomes consumed.

If you wish to find out more information about Diatoms, please see my article here:
How To Get Rid Of Brown Diatom Algae

On the aesthetic front, aragonite sand simply looks good, giving a wonderfully natural appearance.

  • Fine aragonite sand will look silky smooth in your saltwater aquarium, but is easily blown around
  • Fine/medium grain sand will add a nice textured appearance and are the most common
  • Coarse grains are best suited for tanks with very high-flow

What is Live Sand?

Beyond the specific type of sand to use you will also be asked to choose between dry sand and live sand.

Both dry sand and live sand are dredged from the ocean floor, sorted by size, and bagged. Dry sand is then left to dry out and all the bacteria and marine life living within it die. Dry sand is much lighter and more can be shipped for the same price as wet sand.

CaribSea Arag-Alive Fiji Pink Sand
CaribSea Fiji Pink is a Very Popular Sand Grain Size

Live sand is bagged and shipped in a wet state. During bagging a culture of live, but dormant bacteria are added to the bag/water. Upon adding live sand to the aquarium, the dormant bacteria become active immediately and begin converting the ammonia into nitrite.

Using live sand is a way to start and speed up the nitrogen cycle in your saltwater tank.

Some people prefer to let biological filtration happen naturally and use dry sand instead, but the cycle time will be longer. There is no real answer to which type of sand you buy and they are both available.

Most aquarists will purchase the live sand to help speed up the cycle, but if you are installing a very large aquarium, dry sand will be cheaper. A few bags of live sand can then be added to ‘Seed’ the tank. In time the whole sandbed will become colonized with beneficial nitrifying bacteria.

What is the Proper Way of Adding Sand to a Saltwater Aquarium?

No matter where you buy your sand you must automatically assume that it needs to be cleaned before being used. You should never directly add sand from the bag into an aquarium filled with livestock!

Typically, newly bought sand is dusty and needs to be rinsed several times. To do this put each bag of sand in a bucket and:

  • Add salt water if it’s Live Sand – This keeps the bacteria alive – Freshwater will kill saltwater bacteria
  • Add freshwater if it’s dry sand

Swirl the sand around so that the dust and other undesirables float up to the top. Rinse until the water is clear then add the sand by scooping it bit-by-bit onto the bottom pane of the aquarium.

There is nothing wrong with adding the sand directly to a new, empty aquarium, it will just be very murky for the first few days. Just run some filter floss or filter socks and change them regularly to help remove the suspended particulates.

This is the best approach with aquariums over 75 gallons and a large quantity of sand.

NOTE:
Always install your rock aquascape directly on the glass, then add the sand around it! Adding rock on top of sand can cause the rock to topple in the future as crabs, snails, and burrowing fish disturb the sand. A toppling rock at best could kill the coral attached to it, at worst can burst your tank!

To Finish

Although not absolutely necessary for a saltwater aquarium, having a sandbed really impacts the look of your reef tank and will play a role in biological filtration.

We recommend adding two inches of fine/medium grain aragonite sand to the bottom of your saltwater aquarium and using live sand will help to jumpstart the nitrogen cycle.

You can also use coarse sand or have a deeper sand bed but if you go with these options be aware of the potential drawbacks and how you can avoid them.

The sand you chose is only predicated by the look you wish to obtain with your aquarium. Sand or no sand, fine or coarse. There really is no right or wrong.

Further Reading

Richard

Hi, I'm Richard and I have been an avid aquarist for over 30 years with a passion for Saltwater Aquariums. I love to pass on my knowledge to help others get the same amount a pleasure out of this hobby as I do. View my About Me page to find out more about me & my mixed reef aquarium.

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