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How to Choose an Aquarium Substrate: Understanding the Ground Level

When I travel to the Caribbean, I love to look straight down into the crystal clear water to watch the fish swim over the natural rocks, sand, plants and occasional coral. It would be a far different experience to simply look into the water and see fish swimming over the floor of a swimming pool, wouldn’t it?

That’s a major reason why aquarists often love a good substrate; it gives the aquarium that natural look, as though you’ve taken a small piece of the wild and placed it in your own living room. It brings the fish closer to their natural habitat as well, causing your home to merge smoothly with theirs.

But not all substrates are created equal. How do you choose among so many options? Here we will break it down for you in clear and simple terms, guiding you toward the proper choice for your personal aquarium.

Before diving into the specific types, let’s first look more broadly at the qualities of different substrates. The main features to consider are:

  • Particle size
  • Particle shape
  • Particle color
  • Effect on water parameters
  • Presence or absence of nutrients

Let’s examine each of these in turn, and then apply them to the different types of substrates that are available.

Particle Size

Substrate can consist of anything from fine sand to large river rocks. The size of your chosen substrate matters for a few reasons:

  • The larger the substrate material, the easier it is for smaller particles, like waste from fish and plants, to fall through the cracks and accumulate at the bottom over time.
  • The smaller the substrate, the more likely that it will be vacuumed up during tank cleaning.
  • The smaller the substrate, the more likely that it may get caught in the filter or the pump.
  • The smaller the substrate, the easier it is for fish to play in it, move it around, or burrow in it.
  • Small stones that can fit in a fish’ mouth might be a choking hazard if the fish are a type which scavenges the bottom for food like algae.

Particle Shape

The shapes can be smooth like river stones, course or fine like sand, or jagged like gravel.

  • Avoiding gravel or pebbles that contain sharp edges is generally a good idea, especially for bottom dwellers and catfish who do not have protective scales.

Particle Color

Are you looking for earthy colors to complete a natural look? Or do you like to create a more dreamy, fantasy-like world with neon or other bright colors? This choice is mainly an aesthetic one.

  • Darker colors will help light fish to stand out by contrast.
  • Lighter colors will contrast with the darker fish.
  • There is evidence that the color of substrate can affect different fish’ health, stress and color, though seemingly not dramatically.
  • If it makes little difference to the visual appearance that you’re going for, then most fish tend to prefer a darker substrate which is often in accordance with their natural habitat.

Effect on Water Parameters

Some types are inert, meaning that they have no effect on the quality of the water such as pH, KH, GH or nutrients. Others may change the makeup of the water in a way which, depending on your aquarium life, could be beneficial or detrimental. Different substrates can be used to do such things as:

  • Raise or lower pH levels
  • Raise KH and GH levels
  • Assist with the nitrogen cycle by providing enough space for beneficial bacteria to lodge.

Presence or Absence of Nutrients

Depending on whether you have plants, and what types of plants those might be, you may improve your plant life by adding certain types of substrates which provide necessary nutrients.

  • Some plants may thrive simply from liquid fertilizer or root tabs, but others may be best served from the more natural environment of having nutrient-rich soil available in which to take root.

The 5 Types of Substrates

While there are a multitude of different substrates we could discuss, we will take a high-level view by examining them in five broad categories. We will also break down how they exhibit the five qualities listed above.

No Substrate

First off, let’s step back and ask a foundational question: is a substrate even necessary? After all, haven’t we all seen the Cat in the Hat goldfish in a round bowl, containing nothing but water? It’s true, fish don’t exactly need a substrate in their aquarium. But their long-term health is likely to be affected.

Since we cannot break this category down by size, shape, etc., we will simply list the pros and cons of choosing not to use a substrate.


  • Cleaning the tank becomes extremely easy.
  • Uneaten food has nowhere to hide, making it easy (but not as enjoyable) for fish to scavenge along the bottom.
  • There are no extra particles to cloud the water, besides the waste from the fish and uneaten food.
  • It is easier to notice if any eggs have been laid, which makes this particularly attractive for fish breeding.
  • Doesn’t allow a place for parasites to dwell and reproduce, making this the more ideal solution for a quarantine tank.


  • It simply isn’t a reproduction of the fish’ natural habitat.
  • Fish are spooked by their own reflection, so minimizing that reflection by covering the bottom saves the fish from unnecessary stress.
  • The aesthetic appeal generally isn’t there.  A tank lacking a substrate often feels bare-bones like it’s missing something, more like a science experiment than a place to call home.
  • Live plants can’t take root or obtain necessary nutrients.
  • Fish have nothing to interact with: nest builders have nothing to use, fish cannot play with or burrow in it, and they can’t enjoy foraging for food over it.
  • Beneficial bacteria colonies have no solid surface to attach themselves to. Some bacteria consumes ammonia, and other types consume nitrite, helping to eliminate toxic levels of each and produce nitrates for use by the live plants.


The most popular type of substrate would easily be gravel. It’s what we often see offered in the store with a small selection of aquarium equipment. There is also a wide variety of different looks to this type, which makes it fun to imagine just how to take your aquarium look to the next level.


The size of gravel can range anywhere from 2-4 mm on the small end, up to 30 mm or larger on the other end. The larger ones are often river stones, whereas the small and medium sized ones can often be made of quartz, glass or plastic.


Gravel is the main substrate in which you will want to examine and even feel the shape of the stones. Some gravel is very smooth to the touch, like river stones, and some has a more jagged feel. While both are fine, you simply want to avoid sharp edges. Not every stone may be sharp, either, so usually the smoother edged stones are best unless you’re going for a specific look otherwise. In that case, simply remove any stones that might be questionable.


Gravel comes in every color you can imagine. You can find transparent, neon, or even glow-in-the-dark gravel!

Effect on Water Parameters

Gravel is inert, which means it will not affect pH, KH, GH or any other constitution of the water.


Gravel does not contain nutrients needed for plants to thrive, and would therefore need to be supplemented for that purpose.


Looking for those ocean vibes, even if you’ve got a freshwater aquarium? What better way to import the beauty of the sea than by laying down a sandy floor for your fish to enjoy! Just be sure to wash any sand before adding to your fish tank to avoid unnecessary clouding.


Obviously, sand is much smaller on average than gravel; that’s why we love the feel of it beneath our feet! For your aquarium, you can still choose among grain sizes, which typically range from 0.1 mm up to 1 mm. This small size overall allows waste to remain mainly on top rather than getting lodged deep within the substrate, simplifying the cleaning process. Note, however, that the finer sand could potentially cloud the water easier when stirred up.


The smallest sand is considered fine sand, whereas the largest is coarse. Fine sand can compact together over time, trapping anaerobic bacteria and causing a buildup of toxic substances that eventually harm the aquarium life. For fine sand, be sure to turn over the sand regularly to keep any buildup from occurring; otherwise, coarse sand allows for better water circulation and alleviates this potential problem.


Sand ranges from shades of white to tan to black. It may be best to research your fish types to determine if they are more likely to lose color over light or dark sand. White sand often gives the aquarium a larger look, similar to painting a room white instead of black. Black sand can be more comforting to some fish. The contrast between light and dark can also be considered, depending on the color of the fish.

Effect on Water Parameters

There are differing types of sand, which means you should always check first to see what it consists of. Silica sand (quartz) will not affect the water parameters. If the sand contains any pieces of shells, it will release calcium carbonate into the water and raise the pH, KH and GH levels.


Sand is not rich in nutrients for plants, so does nothing for them other than allow them to take root.


Is an aquarium the place to put dirt? Seems odd, doesn’t it? After all, would that muddy the waters?

Actually, soil can be an excellent choice of substrate for your aquarium, especially one containing plant life. Special soil can be purchased for aquariums which is less likely to cloud the water than basic soil from your backyard. For soil, the preferred structure of the substrate is often to blend two layers: the lower layer being the soil, covered and held down by an upper layer like gravel or sand. The main thing to remember is not to mix the substrates together in a blend; keep the layers separate so that they can each effectively do their intended job.


Of course, dirt is going to be very fine, even finer than sand. It still allows beneficial bacteria to lodge itself within for the breaking down of harmful substances like ammonia and nitrites.


Because dirt is so fine, there is of course no chance of there being sharp edges to harm the fish.


Dirt does not vary much in color, but is typically going to appear as black or very dark brown. Clay-based soil may even display a tinge of red.

Effect on Water Parameters

Have you ever heard of Cation Exchange Capacity (CEC)? Don’t let the scientific-sounding term intimidate you. For the purposes of this discussion, all you need to know is that soil has this capacity, and that means that it removes carbonates from the water which reduces pH, KH and GH. You’ll want to keep this in mind when balancing the water parameters needed for your particular fish.


The main purpose of soil in the aquarium is for the nutrients it provides to the plants and the fish. This is an excellent source of nutrients which will cause the aquarium life to thrive.  These nutrients eventually do get largely depleted over time, which means that it will need to undergo one of the following: 1) be allowed to sit without plants for some time while it is re-fertilized from the fish 2) replenished with new soil or 3) supplemented with store-bought fertilizer.

Special Alternatives

Besides gravel, sand and soil, there are a number of other materials which are sometimes used as the substrate for various purposes. Here we will give a brief summary of each.

Crushed Coral

This serves to increase the pH of the water. It also increases the water hardness and alkalinity, which means that pH is less subject to wide fluctuations. It also releases calcium into the water, which is beneficial to fish in many ways including skeletal structure and various physiological processes.


Seashells consist largely of calcium carbonate, which dissolves gradually into the water. This increases pH, hardness, alkalinity and access to calcium for the fish.


Sand made up of aragonite will gradually release calcium carbonate into the water, having the effect of raising pH, hardness, alkalinity and calcium content. Aragonite will raise the pH levels more than crushed coral will, to approximately 8 pH. It is more popular for saltwater aquariums than for freshwater, but it depends on your fish type whether this will be the best option.

Lava Rocks

Most lava rocks will not affect water pH by very much, if at all. Due to its porous texture, it is particularly good at providing a home for beneficial bacteria that absorb ammonia and nitrites. They don’t typically offer much in the way of nutrients. These can have sharp edges that scratch your fish, so it’s important to research your particular fish to determine if it is a bottom feeder or if it likes to burrow in the substrate, and therefore more likely to be injured.


Limestone helps to raise the pH of an aquarium above neutral, though not likely much higher than 8.0 on the high end. It consists of calcium carbonate, which will raise the alkalinity of the water and provide needed calcium for the fish.


As you can see, there are a multitude of choices for substrates to use in the aquarium. You may even be surprised to hear that this is not a comprehensive list! But these are the most common types. Remember that you don’t only have to pick one; in fact, you can layer two or three substrates on top of one another. Just be sure not to mix them together, but keep them separate as best as possible. Simply pick the substrate that is best for your mix of fish, plants and water quality, and then fill your tank floor with 1.5 to 3 inches of the chosen material. Any live plants should be rooted 1-2 inches deep in the substrate. And then you’ve completed the ground floor! You’ve successfully made your fish feel right at home.

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