Aquarium sand: How to pick the right substrate for your saltwater aquarium

Aquarium Sand: Choosing a Saltwater Aquarium Substrate

When you get started with your saltwater aquarium, you have PLENTY of decisions to make. And one of the first you need to decide? Which aquarium sand will you go with to cover the bottom of the tank? Maybe you haven’t considered the number of options out there. Or perhaps you have, and you’re working on narrowing the choices. Either way, I’ve created this handy guide to all things grainy to help you decide on the best aquarium sand choice for your reef tank.

Table of Contents: Aquarium Sand

If you’ve stood in the aisles of your local fish store, you know you have PLENTY of aquarium sand to consider. They come complete with different components, grain sizes, colors, and even functions. Understanding the purpose of each substrate type is critical to get the most out of your saltwater aquarium. You can use the links below to investigate the sand type you’re most interested in. Or, if you haven’t made your decision yet, you can read through the entire article and then hit the shop.

Aquarium Sand: An Introduction

When I first started in the aquarium hobby (a thousand years ago), the decision about what aquarium sand to use was pretty simple. You only had two options: fine sand or coarse sand (basically, crushed coral). You also only had three ways to place it in your tank: a deep sand bed, a shallow sand bed, or a bare bottom.

Now, you still choose your type of substrate and how you plan to aquascape your tank (design the layout). But the increased popularity of the hobby has created more options for aquarists. And I think most people agree it’s better to have MORE options than fewer. That’s where the explosion in aquarium sand comes in handy.

But having TOO many choices can create a different problem: How to pick the right substrate for your saltwater aquarium? To answer (or at least inform) that decision, let’s take a look at what you have available.

Before we get started, though, I want to offer a few quick notes:

  • For the purposes of simplicity (and to avoid repetition), I’m going to consider the words “sand” and “substrate” interchangeable.
  • The substrate is any structural object in the tank (including live rock). But, in this case, please assume the substrate I’m referring to is sand (unless otherwise noted).
Aquarium sand can take many forms

Image by Siim Sepp

Types of Aquarium Sand

Aquarium sand is composed of plenty of different components. Maybe that sounds strange, but if you took samples from beaches around the world and lined up the jars on a table, they wouldn’t look the same. Depending on the barrier reef systems, the local rocks getting weathered by the waves, the invertebrates living in the area, and even the local plant life, you’ll see different textures and colors. And that’s how manufacturers develop types of aquarium sand. They’re working to replicate the natural ocean environments.

The most popular types fall into three main categories:

  • Aragonite
  • Crushed corals
  • Oolite

Aragonite

Aragonite sand is the most commonly recommended aquarium sand in the hobby. It’s available as a fine-grained product that looks like the prototypical sand. (Well, what most people tend to think of as sand when you mention the term) It’s white in color and comes in two crystal shapes (though you probably won’t spend THAT much time examining your sand).

Aragonite sand shows up so commonly because it’s made from the same elements as coral skeletons – calcium carbonate. In fact, the term “aragonite sand” is just a fancy way to say, “sand composed of calcium carbonate.” In water, the sand gradually dissolves, releasing that calcium carbonate into the aquarium, supplying your corals with the building blocks they need.

Some people tout aragonite aquarium sands for that dissolving characteristic, believing they can help buffer your tank. Don’t fall for the hype. While they WILL dissolve, you’re not going to get that buffering assistance. It’s essential to understand the truth about aragonite sand if you decide to pick this one. (No flashy marketing campaigns here)

Aquarium sands vary the same way beaches do

Crushed Coral

Crushed coral aquarium sand, by comparison, are substrates with much larger particles. Instead of fine grains of sand, crushed coral substrates resemble tiny white stones with pieces of crushed-up coral skeletons (where it gets its name). Of course, since you ARE dealing with coral, you’re buying a “bulky” version of aragonite. Sometimes you’ll come across crushed coral with a label of dolomite. Dolomite still contains calcium, but it has the addition of magnesium. (Which reef tanks also benefit from) Either way, you’re going to feed your corals.

Crushed coral aquarium sands provide a more dramatic appearance to your tank, though they can provide some challenges. They’re not as easy for burrowing fish or invertebrates to tunnel through. And without a proper water flow, they can end up turning into a mess. You’ll want to use an undergravel filter if you pick this substrate.

Oolite

Oolite (sometimes called oolitic aquarium sand) is a “fancy” type of fine, spherical aquarium sand. The term “oolite” refers to the way the sand forms. Oolite forms from ooids (shown in the picture below), spherical sand grains formed in concentric layers. By definition, oolites MUST be less than 2mm in diameter. This means oolitic aquarium sand is composed of tiny egg-shaped or spherical grains. They form WITHOUT those calcium carbonate shells you see in aragonite aquarium sand.

Because the term “oolite” refers to size and shape, it doesn’t hurt to check the label of the substrate you’re planning to buy (especially if you’re buying from a non-aquarium hobby source) to confirm the chemical composition. It’s possible to have oolite made from phosphate, hematite, or dolomite. And since they’re abiotic (made without the presence of live animal parts), your tank doesn’t gain as much benefit from these aquarium sands. Dolomite sands are the exception, of course. But make sure you read the label carefully.

Oolite aquarium sand

Image by Mark A. Wilson

Aquarium Sand Colors

You can design your reef tank to have any look you want. And once you settle on the type of aquarium sand you’re looking for, it’s time to choose colors. Every substrate comes with the possibility of a “tint” that will add extra personality to your aquarium. Depending on the style of tank you have in mind, you’ll want to choose the color that works best for you.

Do you want to make your fish, crustaceans, and corals feel at home? Or are you hoping to gain a “glow” with the proper lighting? These are the kinds of questions you want to consider when you look for your aquarium sand colors.

“Traditional” Sand

You can’t go wrong with picking the “traditional” sand color. You’ll find plenty of aquarium sands out there that look like – well, sand. And there’s nothing wrong with that decision. Your fish and invertebrates will cultivate a natural appearance. And (as you can see below) you can even offer the perfect background for camouflage opportunities – with the suitable species. It’s a choice you don’t want to throw out as “boring” or “unimaginative.”

For a natural, functional, and practical substrate, “traditional” aquarium sand is a straightforward choice.

This cuttlefish disappears into the "traditional" aquarium sand

Image by Raul654

White Sand

If you are looking for a brighter look than the “traditional” aquarium sand, you can achieve a more brilliant, whiter color with crushed coral. As mentioned in the section above, this substrate comes from the (bleached) skeletons of dead corals (sounds bleak, doesn’t it?). Out of the bag, crushed coral has an appealing white coloration. However, the brightest color shows when placed against a darker background. And the fresher your aquarium sand? The cleaner the look.

Over time, the white aquarium sand gets covered in a biological film (i.e., bacteria) and becomes clogged with dirt and detritus. So once your aquarium gets up and running, your gravel won’t remain pure white. But don’t worry: that extra color (even if it’s primarily brown…) actually does GOOD things for the health of your tank. (We’ll go into the benefits below)

Black Sand

Black sand sometimes gets marketed under the name “Tahitian Moon Sand.”  This is because Tahiti is known for its black sand beaches. They’re formed as a result of volcanic activity, and lava is rich in obsidian. Black sand forms when hot lava comes in contact with cold ocean water and breaks apart. (Lava – and obsidian – are delicate and prone to fracturing) The color’s natural, with no dyes or coatings to wash off into your tank water.

If you’re planning to grow plants in your reef tank, black aquarium sand is the way to go. It’s rich in nutrients and ideal for promoting plant growth. And there’s no doubt it’ll create a dramatic backdrop for your fish and invertebrates. You just need to trade off the fact that the particles come in on the small side, making cleaning a bit of a chore.

Pink Sand

The pink hue found in pink aquarium sand is actually a combination of white and red. The white side of the equation comes from traditional aragonite, while the red comes from the shells of forams. Forams are tiny, single-celled creatures that create hard, red skeletons. When forams die, they leave their skeletons behind and provide the sand with its pinkish hue.

From a travel and vacation perspective, I think of Bermuda as the most famous place to find pink sand beaches. Some of the commercially available pink aquarium sands come from Fiji, though. Either way, you can add the perfect touch of “tropical” to your reef tank when you go with this look. And your corals benefit from that boost of calcium. It’s a win-win situation.

Functions of Aquarium Sand

Some people choose to keep an aquarium with a bare bottom. It’s a clean look and easy to maintain. But with smooth glass in your tank, you’re missing out on the advantages of aquarium sands. And that’s one of the reasons hobbyists spend so much time picking and choosing their substrates. It’s more than following a traditional pattern (even commercial tanks add sand). If you want a thriving, healthy mini-ecosystem, you turn to aquarium sands.

Aesthetics

I’m not sure if you’re like me, but I feel this pressure to know what everything in my aquarium does. And that includes aquarium sands. One of the most basic functions of the substrate is to provide an aesthetic look and feel to a saltwater aquarium. It’s the perfect complement to your reef tank aquascaping. The right choice of sand can help you achieve the look you’re going for. (After all, creating a beautiful reef tank is what the hobby is all about)

I encourage you to consider the aesthetics of aquarium sand as equal or more important than the other functions. You can (and should) read the section on denitrification and think, “Wow – what a great benefit of keeping a deep sand bed!” But if you’re not crazy about that big, nasty-looking sand bed, don’t do it. You can find other ways to control the nitrogen in your tank.

Use aquarium sands to create the look you want. That’s the first and foremost function of the substrate. The other benefits you get are icing on the cake.

Aquarium sands provide surface area for beneficial bacteria

Surface Area

Biological filtration in a saltwater aquarium is performed by the bacteria growing on the live rock and in the live sand in your aquarium. Aquarium sands with plenty of surface area create the perfect home for those biologically beneficial bacteria to grow. All of the individual faces of each grain of sand are places for bacteria to colonize. So the finer your grain of sand, the more room you have for a biological filter. It works with crushed coral, too. All of the individual nooks and crannies of those broken pieces attract bacteria. In no time, you have an extra filter going to work for you.

But that doesn’t mean that you HAVE to have a sand bed to create a biological filter. Plenty of aquarists are successful in keeping reef tanks with no aquarium sands. Bare-bottom tanks manage the same technique, courtesy of their live rock displays. You may not see the same level of beneficial bacterial growth (it’s a math thing: less surface area is available), but the theory continues to function.

It’s up to you whether you want the advantage or not.

Burrowing Substrate

Aquarium sands often serve as the home for many ocean creatures: engineer gobies, jawfish, shrimp, snails, crabs, and worms (to name a few). And you’ll find plenty of animals that sift through the sand looking for a quick snack (or a full meal). And having the proper sand bed is critical for keeping these creatures. They need the right size of a sand grain to support their tunneling habits. And, of course, all of that surface area and available “biology” in the sand is a given.

You can spend HOURS watching your fish and invertebrates go to work in the aquarium sand. It’s entertaining, and it takes the proper skill to keep them happy and safe. Plus – if you’re lucky – you’ll have a few unexpected and interesting guests take up residence in your sandbed. Hitchhikers on your live rock often move into the sand and take up residence. (Some you’ll appreciate, others you may not. It’s a mixed bag) Check things out with a red light at night.

Garden eels enjoy deep beds of aquarium sand

Denitrification

If you have a sand bed of at least 6 inches (15.2cm), you have what is called a deep sand bed. The advantage of having a deep sand bed is that deep oxygen can’t reach the bottom layer. There are anaerobic bacteria that remove nitrates from the water living in those anoxic (oxygen-free) depths of the sand. So an aquarium with a deep sand bed (either in the display or in a refugium) can not only turn ammonia into nitrites and nitrates (following the nitrogen cycle), but it can also remove the nitrates from the water column altogether (denitrification).

Aquarium sands give you better control over the cycling of your reef tank. And all you have to do is set up the proper depth! You’re already getting a biological filter out of the mix, but now you have a chance to give your nitrogen cycle a tiny “goose.” It’s one of the reasons hobbyists often turn to deep sand beds – assuming it isn’t needed for one of their favorite fish or crustacean species.

Defining “Dry” Sand and Live Sand

One aspect of purchasing aquarium sand that can trip people up is whether to buy “dry” sand or live sand. They look the same in the bag, and they come in the same types. So what’s the difference between the two? (Well, other than the label and price)

As mentioned in the previous section, a critical function of aquarium sands is to provide surface area for bacteria to grow and generate a biological filter. Those bacteria start to grow after you set up your aquarium. Once your tank completely cycles, there’s a relatively robust population of bacteria growing on every surface under the water. But cycling takes time, and you’d like to enjoy the beauty of your reef tank NOW.

One way to shortcut cycling time and grow your bacteria naturally is to purchase live sand. Live sand is aquarium sand already colonized with helpful bacteria. The sand is packaged and shipped “wet.” When you open the package, it contains biologically active bacteria (and the potential for some other organisms). All you need to do is add it to your tank and watch your ammonia and nitrite levels drop.

Live sand gets expensive compared with “dry” sand (which doesn’t contain any of those bacteria). But if you must get your aquarium up, running, and filled with livestock immediately, it’s a time-saving product. Of course, ALL sand eventually turns into live sand. So if you want to save some pennies, pick up the “dry” sand, add it to your tank, and wait.

Calculating how much aquarium sand to purchase depends on several things

How Much Aquarium Sand Do You Need?

Another popular question for new hobbyists is how much aquarium sand is needed for a reef tank? And the age-old answer is – it depends. I know, not a satisfying answer. But it depends on what you’re trying to achieve:

  • If your goal is to look for aesthetics: Follow your intuition and go for the depth of sand that looks right to you.
  • If you are a rule follower and want me to give you a rule-of-thumb: Start by aiming for 1-2 inches (2.5-5cm) in depth. That seems to be a popular and aesthetically pleasing depth to a lot of people.
  • If you want a deep sand bed: The rule-of-thumb is at least 6 inches (15.2cm) deep. If your tank is only 18 inches (45.7cm) tall, that’s a lot of sand. You may want to reconsider.

How Much Sand per Gallon?

Now that you’ve settled on how deep a sand bed you’re aiming for, how do you decide how much aquarium sand to buy? To figure it out, you need to calculate the volume of the base of your aquarium (1-2 inches/2.5-5cm across the bottom). Then you do a conversion to figure out the weight of sand you need to fill that volume. It’s not complicated math, but it’s a little involved.

But why do the math when you can use a “close-enough” rule-of-thumb? To get approximately a 1-inch (2.5cm) layer of sand, you need 1 pound (0.45kg) of sand per gallon (3.8L). So if you have a 10-gallon tank (38L), shoot for about 10 pounds (4.5kg) of aquarium sand. If you have a 55-gallon (208L) tank, aim for about 55 pounds (25kg) of sand.

Now, actual results will vary based on the shape of your aquarium and the size and shape of the grains of sand. (Coarse aquarium sand takes up more room, and fine sand takes up less space). But if you start with that idea in mind, you can get close to your desired depth. Then you can tweak it higher or lower based on how it looks in your tank.

I’m the type of person where “close enough is good enough.” But if you’re the type that needs an exact answer, I encourage you to check out this online calculator. It looks intimidating, but it’s relatively easy to use and will tell you exactly how much aquarium sand you’ll need.

Adding Aquarium Sand to Your Aquarium

Now that you’re ready with your aquarium sand, it’s time to get it into the tank – without upsetting anything. And while it sounds easy (just dump it in, right?), there’s a process to tackling this step. If you don’t want to make a mess out of things, you’ll want to follow these guidelines.

First, let’s assume you have a brand new tank with NOTHING inside. The first step is to clean the aquarium sand. Believe it or not, the sand you buy (if “dry” sand) tends to be full of dust. If you put that dust into your tank, you’ll end up with a horribly cloudy and ugly mess.

  1. To clean the sand, simply pour some into a bucket. Fill the bucket part-way with water, reach in, and stir the sand with your hand. The water in the bucket will look cloudy and gross. Keep rinsing and mixing the sand until the water shows clean.
  2. Gently scoop the sand with a plastic cup or sturdy (non-breakable) bowl, and add the sand to the bottom of the tank. You want to be careful not to dump large quantities in a single area, so you don’t stress and break the glass.
  3. Continue until the tank bottom’s covered to your chosen depth.

If your tank is already set up and you want to add aquarium sand, the technique is slightly different. You still need to clean the sand (that dust can still create problems and clog your filters). But you need to work a different way.

  1. Scoop the sand into a cup or bowl, lowering the cup all the way to the BOTTOM of the tank and then gently turning it over. If you dump the sand in from the top of the tank, your water will end up cloudy.
  2. Continue to work as slow as possible, taking your cup or bowl to the bottom of the tank.

The YouTube video below provides a visual on how to clean and add sand to your aquarium. (Yes, the tank appears to be freshwater. But I wouldn’t do anything differently than shown)

Choosing the Best Aquarium Sand For a Reef Tank

Picking the best aquarium sand for a reef tank is as simple as figuring out what you want and finding the substrate that works for that design. Figure out the look you want, and purchase a type of aquarium sand that mimics that look. Then decide on the depth you want, follow the pound-per-gallon estimate, and add that much sand. Voila! You’ve settled on the best aquarium sand for the job!

Try to avoid the “no-mans-land” between 2-6 inches (5-15.2cm). You won’t get the denitrification process here, as it’s too shallow to create an anoxic zone. Instead, you’ll find yourself struggling with an overgrowth of UNHELPFUL bacteria (or other organisms). You also want to pick an inert sand (aragonite or silicate). And if you have more money than time, buy live sand instead of dry sand.

The reality is, it’s hard to go wrong when picking aquarium sand. Whatever works best for YOU is the correct choice. Your fish, corals, and other invertebrates will stay happy. And your tank? It’ll look fantastic.

For More Information

Obviously, you’re interested in aquarium sands. You wouldn’t have stuck around this long otherwise. (Unless, of course, you’re a devoted fan of the site) So let’s make sure you’re getting everything you need in terms of information.

How about the ultimate guide to aquarium sands? This YouTube video is on the long side, but it provides plenty of helpful information:

 

Or maybe you’re curious about some of those creatures you can add to your tank with the appropriate substrate? Let’s take a look:

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