A Comprehensive Guide to Aquarium Substrate Choices

Aquarium substrate is one of the most essential, yet overlooked, elements when it comes to keeping fish. It affects the psychological needs of your tank’s inhabitants, the water parameters, and influences plant growth.

In this article, we’ll take a closer look at the aquarium substrate, and answer questions like:

• What is an aquarium substrate?
• How does it affect the aquarium?
• What are the best options?
• What are the most affordable options?
• Do I need to change the aquarium substrate?
• How do I clean aquarium substrate?
• And many more!

Let’s get into it.

What Is Aquarium Substrate?

The aquarium substrate is the substance you place at the bottom of a fish tank. It can be many things, ranging from glass pebbles to coco coir or even sand. 

Why Should You Use A Substrate?

There are many reasons to use a substrate in your aquarium. To name just a few:

• It gives you a sturdy base to which you can anchor any decorations that you might want to incorporate.
• Good substrates add a layer of biological filtration to your aquarium, helping to balance and control the nitrate and ammonia cycles.
• Many bottom feeders need an appropriate substrate to feed properly and stay healthy.
• The substrate plays an essential role in the courtship rituals and breeding cycles of many cichlids and other fish.
• Some fish need a specific type of substrate to help them feel safe and to reduce stress.
• Substrates can help make the tank easier to clean by giving detritus a place to settle, rather than circulating throughout the aquarium. 
• An appropriate aquarium substrate will allow you to use undergravel filtration.
• It makes the aquarium look nicer, and provides a more natural habitat for the fish.

Is It Okay Not To Use A Substrate In Your Aquarium?

In some circumstances. Fishkeepers call tanks without substrate bare-bottom tanks.

Quarantine tanks are one example. Aquarists keep these tanks substrate-free to prevent parasites and diseases from making their home in the substrate. It also facilitates easy cleaning.

Some keepers also like to keep messy or carnivorous fish in a bare-bottom aquarium. We don’t recommend this, because the substrate can help keep the nitrite and nitrate levels in check. It also offers a safe space to keep unobtrusive bottom feeders. 

What Are The Main Types Of Aquarium Substrates?

There are many different types of aquarium substrates that you can use. Some of the most common types include:

• Sand – One of the most commonly used, especially for aquascapes and fish that dig
• Gravel – Popular among beginners and for unplanted tanks 
• Pebbles / River Stones – Rarely used, except for large fish and to prevent digging
• Peat Moss/Coco Coir – Mainly used for specific fish that require soft water and high pH
• Clay and Clay Pellets – Commonly used in planted tanks or for large fish

Commercial Aquarium Substrates

Commercially produced aquarium substrates run the gambit from basic to advanced solutions. These include common substrates like gravel and sand, but more specifically include special mixes like:

• Aquasoils
• Aquarium plant mixed
• Baked and fused clay products
• Mineralized or mineral-infused substrate mixes

Different brands may have different names for their mixes. For the most part, the mixes are similar and focus on making them great for plants. Clay products, and commercial crushed stone, are the exception, being singular elements intended for mixing or layering in the aquarium.

Considerations When Choosing A Substrate For Your Aquarium

There are many different things that you need to consider before choosing an aquarium substrate. The following sections will take a closer look at specific considerations when choosing a substrate.

Inert Vs. Active Substrates

There are two main types of substrate for aquariums. 

Inert substrates are derived from rocks and minerals. They break down extremely slowly, or not at all. They also have little to no effect on the water parameters.

Active substrates may be an organic material, like coco coir or sphagnum moss, or they may be inorganic. They affect water parameters, which is often why aquarists use them.

Some substrates straddle the lines between the two categories. Limestone, for instance, is considered an inert substrate but it affects the water hardness.

If you’re going for a planted bioscape or another ecosystem-style tank setup, then you’ll probably want an active substrate for the biological possibility. If you’re going for a display-style setup with little biological interaction, inert substrates will work just fine.

Water Hardness

Most types of fish prefer a specific range of water hardness. Since the substrate covers the entire bottom of the tank, it can play a significant role. Why is that? Many mineral-based substrates affect water hardness due to soluble elements in their structure. Examples include limestone and aragonite, which make water harder.

By affecting the pH, substrates like coco coir and peat moss may also have a minor effect on the water hardness. 

Water pH

Different substrates may also alter the pH of the water through interaction with the water and each other. For instance, coco coir and peat moss both contain tannins. When tannins get released into the water, they increase the pH, making the water more acidic.

The appropriate pH is vital for healthy fish.

Weight

Substrates vary significantly in weight, and this element bears plenty of consideration. While the substrate’s weight generally has little effect on the fish, it has a great effect on the aquarium itself.

Not all tanks and, more specifically, tank stands are up to pounds and pounds of weight from the substrate, in addition to that of the water. 

Inhabitants

What you’re keeping in your tank can have a significant impact on the type of substrate you choose. 

For instance, apple snails do best with a substrate containing some sort of calcium. Pit-building cichlids require sand or large pebbles (depending on their size) to build the nests where they court females and lay eggs. Always build your tank around the animals you plan on getting, and do careful research so you can give them an appropriate substrate.

Nutrient Retention for Plants (Change Exchange Capacity)

If you’re planning on doing a planted aquarium, you need a substrate with a good CEC. The CEC measures how good a substrate is at receiving nutrients and then releasing them to plants. 

Gravel has a fantastic CEC, while sand has a poor one. You can also buy sculpted soil mixes specifically for plants, and change them out every couple of months.

Cleaning

Cleaning the tank is easier with some substrate types. Larger materials are generally easier to maintain than smaller ones. 

I.E. River stones, pebbles, and gravel are easy to clean because they have large spaces between them. Sand, coco peat, and soil mixes are harder to clean because they’re easier to siphon out of the tank accidentally.

Substrate Choices: Pros And Cons

As you might expect, not every aquarium substrate is equally great. Let’s take a closer look at the pros and cons of each.

Sand

Sand is one of the most commonly used aquarium substrate choices. Its advantages including being:

• Inexpensive
• Readily available
• Available in a plethora of colors
• Easy to install and use

The disadvantages include:

• It can be difficult to clean
• It builds up an abundance of nutrients over time, leading to ammonia and nitrate spikes

Gravel

Gravel is the first aquarium substrate that most fishkeepers use. Whenever you buy an aquarium kit or ask for a substrate at a pet store, they typically give you gravel.

The pros of this substrate type include that it’s:

• Inert
• Easy to use
• Readily available
• Great Change Exchange Capacity (CEC)

Clay Substrates

Clay substrates like LECA and other clay pellets are common because they’re readily available. Their advantages include being:

• Readily available
• Generally affordable
• Easy to saturate with nutrients
• Better porosity and airflow than substrates like sand

Disadvantages include:

• Typically being manmade – not entirely natural
• Having a shelflife – nutrient retention dwindles with time
• High in iron, and not always balanced with other nutrients

Peat Moss and Coco Coir

Peat moss and coco coir have similar effects and traits but come from different places. Peat moss is harvested from bogs, while coco coir is manmade from the hairs of coconuts.

Advantages include:

• Containing tannins for fish that need them
• Raising the water’s acidity level
• Acts as a water softener by neutralizing dissolved minerals

Disadvantages include:

• It’s hard to calculate how much you need
• Not all plants like growing in these substrates
• Can be difficult to clean, and must be replaced frequently

Soil Mixes

If you’re planning a Dutch aquarium (a fish tank that only contains plants) or a heavily planted aquarium, then soil mixes are worth considering.

Their advantages include that:

• They’re perfectly formulated for growing plants
• They have high nutrient retention levels
• You can buy mixes for both acidic and alkaline aquariums

The disadvantages include:

• A relatively short shelflife
• That they’re quite hard to keep clean
• They can be expensive

More About Sand

Sand is a common aquarium substrate choice and, for many species, is a fairly natural choice. However, this substrate has some special considerations that you’re unlikely to encounter with many other substrates.

Useful And Affordable Types Of Sand

Sand is sand, right? Unfortunately, nothing could be further from the truth. Here are some great options for a fish tank:

• Black Diamond Sandblasting Sand – Polls have found that many aquarists like to use Sandblasting Sand for their aquarium. Partially due to its sleek black coloration, and partially because it’s inexpensive. Only buy the 20/40 grit, as it’s the best consistency for the fish.
• Pool Filter Sand – Because filter sand is used for human swimming pools and fish ponds alike, it’s usually safe for use in an aquarium. However, ensure it isn’t pre-salted for use in saltwater swimming pools.
• River Sand – River sand from unpolluted rivers is an ideal substrate for aquariums. You’ll also need to boil the sand to ensure that no pathogens or parasites are hiding in it.
• Untreated Play Sand – Clean play sand that hasn’t been salted or treated will work quite well. Since it’s manufactured for use in children’s sand pits, where it may get eaten, it’s generally pure.

Types of Sand You Should Never Use

• Beach Sand – Never use beach sand for a freshwater aquarium. Even with rigorous preparation techniques, the sand will retain some salt. This will quickly turn your freshwater brackish and hard. It may also contain traces of pollutants.
• Building Sand – Unless you know exactly where building sand comes from, this sand isn’t usually viable for tanks. Companies often quarry sand around mines, which means this sand may be full of pollutants from the mining process. It may also be too coarse for your fish’s sensitive skin.
• Salted Sandpit Sand – Treated sand for sandpits isn’t viable as aquarium substrate. It often contains salt, which can kill your freshwater fish. Some companies may also treat it with other substances.

Dealing With Gas Pockets

The formation of gas pockets is one of the main problems people encounter when using sand as a substrate in the aquarium. Because sand particles fit together so closely and form such a dense layer, detritus can become trapped between them.

As the debris starts to decompose, it forms pockets of ammonia and other harmful gasses. If these pockets somehow become exposed to the water at large, it can have lethal consequences for your fish. 

So, how do you prevent or solve the buildup of gas pockets? Let’s take a look:

1. Keep bottom feeders like corydoras, snails, kuhlii loaches, and Ancistrus catfish; They’ll prevent food from building up on the bottom.
2. Keep fish like kuhlii loaches that will burrow into the substrate, stirring it and preventing gas buildup.
3. Stir the substrate regularly with chopsticks or a small net to prevent gas from forming pockets in the sand. You can then catch any debris in a net, as long as you time it well to prevent catching grains of sand.  

Adding Substrate To Your Aquarium

The basics of adding substrate to an aquarium remain fairly consistent, irrespective of what substrate you use. Simply:

1. Rinse the substrate thoroughly to remove debris and contaminants.
2. Ensure that the bottom of the tank is clean (remember to wash and rinse it thoroughly if it’s a new aquarium).
3. Use a small scoop, large spoon, or a container to insert the substrate, spreading it across the entire base of the aquarium.
4. Once the entire bottom is covered, start adding extra material to any areas that you’d like higher than the rest. 

Using Substrate Supports

In some cases, you’ll want to incorporate heightened areas or use specific substrates in part of the aquarium. In these cases, substrate supports can be handy. 

Essentially, they’re miniature retaining walls for the substrate that anchor to the bottom of the tank, or into the base of the substrate. You can buy them easily online, or look for the design and make a DIY version out of non-toxic materials.

Layering Substrates

Under certain circumstances, you’ll need to layer substrates. Typically when you’re doing planted aquariums. 

When you’re using this approach, insert your substrate as you usually would, one layer at a time. Start with the largest substrate type, and end with the smallest.

Do You Need To Change Substrate?

Many new fishkeepers aren’t sure whether they’d need to change the substrate in their aquarium. For gravel, the most commonly used substrate, you won’t need to change it out as long as you keep it clean. 

However, that’s only because gravel is an inert substrate. If you’re using some kind of active substrate, then you’ll probably have to replace it periodically. That’s because the active ingredients in many substrates can diminish over time. 

For example, coco coir, peat moss, and sphagnum are excellent for raising the pH in a fish tank. As time goes by, their acidity leaches into the water. 

Every time you do a partial water change, you remove some of that acidity. Eventually, the substrate has no more acidity or tannins that can leach into the water. At this point, you need to start replacing the substrate. 

How Often Should You Change Substrate?

It depends on the substrate in question. Let’s try to put this in some sort of perspective:

• Gravel – Never, provided you keep it clean.
• Sand – Every two to five years.
• Fired Clay Substrates (like LECA) – Every two to four years. 
• Crushed Coral, Sandstone, or marble – Every year or two. 
• Peat Moss – Once a month, to once every three months.
• Aquasoil and most other substrate mixes intended for plants – Every six to 12 months. 

There are too many substrate options and mixes out there to list. For the best results, read up about your specific brand and type of substrate. The best aquarium substrate manufacturers often mention the substrate’s working life on the packaging.

Some More Things to Consider

Does The Tank Size Affect Your Substrate Choice?

No, tank size doesn’t affect substrate choice. Naturally, larger tanks can take larger substrates, but that’s about the extent of it. 

Do You Need To Sterilize Substrate? Rinse it?

In most cases, you won’t need to sterilize your substrate. The main exceptions are if you harvest a substrate from the wild, or use a substrate that has been wild-harvested. 

If so, you’ll want to place the substrate in boiling water for at least sixty seconds to kill off potential diseases and parasites.

Rinsing is essential, for any new aquarium substrate. Options like peat moss may turn your water entirely brown if left unrinsed, gravel can turn the water cloudy, etc. It’s just not worth the effort it will take to get the water clean after not rinsing substrate.

How Do You Fill An Aquarium Without Disturbing The Substrate?

Most veteran fishkeepers have some or other technique for adding water without disturbing the substrate. Typically, that involves using a tall glass, placed upon a saucer that rests on the substrate. 

By slowly filling the glass till it overflows, you have control over how quickly the water flows. The saucer helps to disperse the water so that it only impacts the small area surrounding the saucer.

Once the water has reached the level just below the glass’s lip, you can use a siphon or small container to keep adding water to the surface of the tank. Always allow the water to flow gently onto the surface, instead of pouring wildly. This will keep the substrate in place.

Does An Aquarium Have To Be Perfectly Level?

While slight variations in the level of your aquarium are okay, you want it to be as level as possible. If the base of the tank isn’t level enough, it puts too much strain on parts of your aquarium and can lead to cracking or warping.

Can Substrate Be Too Deep?

Yes! Preferably, your substrate should be no more than two inches deep. Deeper substrate is harder to clean and can become a breeding ground for pathogens.

Changing Substrate In An Established Aquarium

Once your aquarium is established, you need to be careful when changing the substrate. Changing a large percentage of the substrate at once can lead to a serious shift in nitrite and nitrate levels. 

Only change around one-sixteenth to one-eighth of the substrate weekly. At most, you should change a quarter of the water each time, to prevent a sudden nitrate spike.

Final Thoughts

Choosing the right aquarium substrate depends on what your plan for the tank is. Most substrates are good under the right conditions.

If you’re planning a planted aquarium, consider layering substrates with high nutrient retention. But, if you only want to keep fish, choose something easy to clean, like gravel or pebbles.

We hope that the considerations and attributes we’ve listed help you find the perfect substrate for your aquarium!

If you have any questions, feel free to contact us and we’ll do our best to get back to you!