Aquarium substrate is the material aquarists use to cover the bottom of their aquarium(s). Some examples of substrates are gravel, sand, river rocks, and crushed coral.
A substrate benefits most fish because it creates a home for beneficial bacteria. It also creates a home for the fish and encourages natural behaviors.
Some substrates contain nutrients that help plants grow.
Others are simply nice to look at and make the aquarium more appealing.
Some fish need specific types of substrate, like aragonite sand or crushed coral. These types of substrates alter the chemistry of the water. They create the ideal environment for certain species, like marine fish and cichlids.
Other fish do not care about their substrate and hardly notice it’s there. Aquarists are more likely to pick substrates that are decorative in these cases. Substrates like gravel and regular sand rarely cause a change in water chemistry.
Besides knowing the species’ needs, hobbyists must consider whether they want live plants.
They must also consider how much substrate they need and how expensive it is.
They must think about how often they’ll need to clean the substrate and how difficult it may be.
Appearance is important, though far less crucial than all other considerations.
Some aquarists choose not to use any substrate, though it’s rare. Bare-bottom tanks are more common for extra-large fish and hospital and breeding tanks.
Types Of Substrate
Aquarium gravel is the most popular substrate in the freshwater aquarium industry. It comes in many sizes and colors and is cheaper than other substrates.
River rocks provide a smooth, natural look, while clay substrate is best for plants. Peat, laterite, and vermiculite are also good for plants, though hobbyists rarely use them on their own.
Crushed coral and aragonite sand are more common in marine aquariums. They alter the water chemistry for a higher pH.
Regular sand is suitable for soft-bellied fish, like loaches and cory cats.
Aquarium Gravel
Gravel is most common in freshwater tanks and comes in many shapes, sizes, and colors.
Some rocks are smaller, soft, and round for easy digging.
Smaller and softer gravel is better for fish with sensitive skin that like to dig, as it isn’t likely to cause injury.
Other gravel is larger, harder, and misshapen.
These are usually best for fish that don’t live at the bottom of the tank since they can cause injury.
River Rocks
River rocks are like gravel, though they are usually far larger.
Most companies smooth the rocks out and polish them to make them shiny. They come in many colors, though they are usually natural looking. They differ from gravel which is often artificially colored.
Some river rocks come entirely natural, without any polishing.
River Rocks Are Harder To Clean
The trouble with river rocks is their size, which makes them difficult to clean.
There is much space between each rock, creating plenty of room for waste to accumulate. They are also heavier to push around with a gravel vacuum.
Layer River Rocks With Gravel
Some aquarists combine river rocks with gravel. Mixing the substrates combats waste accumulation and looks nicer.
The gravel is denser and settles to the bottom of the tank. It creates a thick layer where the waste can’t accumulate as readily. The less-dense river rock sits on top.
The two layers create a nice effect since the gravel completely covers the bottom of the tank. Using only river rock often results in the exposure of parts of the glass tank bottom.
Marbles
Marbles are a similar, though unnatural, option similar to river rocks.
Breeders of egg-scattering fish often use marbles as a substrate. The eggs easily fall into the spaces between the marbles. They stay safe inside those spaces until hatching.
Regular Sand
Regular, dry aquarium sand is okay for both fresh and saltwater tanks. It is popular because it comes in natural colors. Many brands also provide artificial, vibrant colors.
Sand is best for fish that like to dig or have sensitive bellies, like cory cats, loaches, and some cichlids.
It is easy to clean since the waste sits on top of it, though it can cause problems with the filter.
Aragonite Sand
Aragonite is a calcium carbonate mineral. It naturally raises the pH and alkalinity of the water. Its altering capabilities make it popular in marine, reef, and cichlid tanks.
Aragonite usually comes in sand form, creating a beautiful, natural ocean-like landscape.
Some aragonite sands are sold as “live sands” for marine and reef tanks.
Live sand contains living organisms that help jump-start the nitrogen cycle. They also keep the tank biologically stable once the tank is established.
These sands usually come in a bag of saltwater to keep the organisms alive.
Aquarists can only use live sand in marine and reef tanks. The tiny organisms that live in the sand cannot live in freshwater.
Crushed Coral
Crushed coral comprises tiny bits and pieces of coral.
Coral makes a good substrate for marine and reef tanks. It raises the pH and alkalinity to the levels that saltwater fish need.
Some aquarists use it in brackish water and cichlid tanks for the same reasons.
Crushed coral contains high levels of calcium. Calcium is essential to the health of the skeletal structures of some animals. These animals include corals, mollusks, and starfish, among others.
Crushed coral alters the water similarly to aragonite sand. Crushed coral is larger and coarser, which gives the tank a different aesthetic.
Clay Soil & Other Substrates Great For Plants
Clay soil, like Seachem Flourite, is excellent for freshwater planted tanks. The soil stores nutrients that help with plant growth, and the plants absorb them.
The porous clay is also easy for plant roots to grab onto.
Other beneficial soils for plant growth are peat, vermiculite, and laterite.
The substrates that are best for plants are often dusty or messy. They can be challenging to clean. The brownish-black dust permeates the water once disturbed.
Some soils, like peat, are very light and won’t stay at the bottom of the aquarium without some help.
Layering these soils with other substrates is best rather than using them alone. Layer them with gravel, clay soil, or sand.
The good-for-plants substrate usually goes on the bottom with gravel on top.
Layering helps keep lighter soil at the bottom of the tank and keeps the water cleaner.
Peat
Peat is a dark brown, clay-like material that looks like soil. It is common in bogs and fens and results partially from the decomposition of vegetation.
Peat is very common for gardening because it’s rich in minerals beneficial for plants. It also lowers the acidity of water, meaning it’s not good for cichlid tanks.
Vermiculite
Vermiculite is a hard mineral that plant keepers often mix into their potting soil. It holds many beneficial minerals that help with plant growth and health.
Laterite
Laterite is a clay that is hard when dry, though it stays soft when wet. It is reddish, which can add an attractive hue to planted tanks.
It is excellent at absorbing, storing, and releasing nutrients.
Not Using Any Substrate
Foregoing a substrate is a less common option. A tank without a substrate doesn’t look as nice and can be stressful for fish.
There are some scenarios where a bare-bottom aquarium is beneficial.
Giant Fish
Aquarists that keep massive fish sometimes prefer bare-bottom tanks. They are easier to clean and are cheaper.
Fish like Arowanas need hundreds of gallons of water. The amount of gravel needed for this size aquarium is astronomical and expensive.
These giant fish also produce much more waste than smaller fish. Bare-bottom tanks are far easier to clean than those with a substrate.
Hospital & Breeding Tanks
Bare-bottom tanks are common in breeding and hospital tanks. These tanks are far easier to clean and medicate with no substrate.
Catching fish is more manageable when there is no substrate, particularly fish fry.
Why Aquarium Substrate Is Important
Aquarium substrate is crucial to many aquariums because it keeps the tank healthy.
It provides a home to beneficial bacteria and bottom-dwelling critters. It also helps plants to grow quicker and healthier.
Besides the many health benefits, aquarium substrate simply looks nice.
It Acts As A Home For Beneficial Bacteria
One of the primary benefits of the substrate is that it acts as a home for beneficial bacteria.
Good bacteria are crucial in the nitrogen cycle. The nitrogen cycle is the process every tank must undergo to become safe.
In short, waste in an aquarium breaks down into ammonia. Ammonia further breaks down into nitrites, which further break down into nitrates.
Without beneficial bacteria, those toxins rapidly rise to dangerous levels. Beneficial bacteria help limit those toxins. Reducing the toxins keeps the environment safe for fish and inverts.
Besides the filter, gravel is the main home for beneficial bacteria. The bacteria hide within the tiny nooks and crannies within the gravel.
With gravel, there is more space for the bacteria to accumulate. It is easier to keep toxin levels under control with a substrate.
It Acts As A Home For Some Fish & Inverts
Another primary benefit of substrate is that it calms fish and makes them feel safe.
Fish have substrate in their natural environment, whether soil, rocks, or sand. Mimicking a fish’s natural home makes them feel more at ease.
Hiding Eggs
Many fish lay their eggs on the substrate since it keeps them hidden and safe from predators. Mottled substrates, like pebbles, are particularly good at hiding delicate fish eggs. The eggs may fall between the substrate if large enough, offering extra protection.
Digging
Others, like cory cats and cichlids, are diggers that look for their food in the substrate. Bottom-dwelling fish often use the substrate as camouflage and as a hiding place.
Preventing Reflections
A tank without a substrate covering the bottom creates reflections. Fish dislike seeing their reflections; they don’t understand what a reflection is.
A reflection looks like another fish, which they might perceive as a threat.
Substrates Are Beneficial For Plants
Substrates, especially fine ones, help plants take root and grow. Plants need something for their roots to hold onto; most can’t grow in an aquarium without gravel.
Many substrates contain natural nutrients that help plants thrive and grow faster. These substrates include clay soil, peat, and vermiculite.
Substrates Look Nice
Substrates simply make the tank look nice. Regardless of a hobbyist’s aesthetic preference, there are many options.
Aquarists often choose white or black because it contrasts with the fish’s vivid colors.
Kids often like bright, painted gravel that makes the aquarium pop.
Others prefer natural stones or pebbles that reflect the fish’s natural environment.
How To Choose The Right Substrate
The first thing to consider when buying a substrate is which kind is best for the fish. Some fish prefer specific substrates over others and can even get hurt by certain sorts.
Figure out which type of substrate is best for your fish first. Then look at the size and appearance of the gravel.
Fish Species
The type of fish an aquarist keeps often determines the substrate they can have.
For example, cory cats and cichlids like to dig. Soft gravel or sand is best to encourage their natural behaviors and to prevent injury.
Marine species do best with aragonite substrates and crushed coral. They naturally increase pH and alkalinity. They reduce the work the aquarist must do to keep the water parameters safe.
Cichlids also do well with aragonite and crushed coral. Like marine species, they need higher levels of pH and alkalinity.
Mid to top-dwelling fish rarely care about the kind of substrate in their tank. Hobbyists can focus more on the appearance of the substrate and how they want their aquarium to look.
Whether You Have Live Plants
Aquarists planning to keep live plants want a substrate suited for the plant’s health. Such substrates generally comprise some kind of clay or clay-like soil.
Live plants can grow in regular gravel if the gravel is small enough for the roots to grip.
Nutrient additives are necessary for most plants. Since gravel doesn’t provide nutrients to plants, liquid-based nutrients are essential.
Sand is sometimes okay for plants, though it’s hard for plant roots to grip. Most hobbyists that want to use sand layer it over aquarium soil to give the plants something to hold onto.
How Much Substrate You Need
An aquarium should have a layer of gravel about 2 in (5.1 cm) deep. A layer of sand should be about 1-1.5 in (2.5-3.8 cm) deep.
While not precise, the general recommendation is to use 1-2 pounds of gravel for every gallon of water.
The exact amount of gravel needed may vary depending on the surface area of the aquarium.
Shallower and longer tanks have more surface area and need more gravel.
Deeper tanks with less length need less gravel.
More than 2 in (5.1 cm) of gravel is unnecessary and can be problematic.
The more gravel in the aquarium, the less space there is for the fish to swim.
The too-deep substrate can also create areas where there is little oxygen. The oxygen-deprived spaces eventually release toxins into the water.
Substrate Size
The gravel size is an essential focus for aquarists as it impacts the health of the aquarium.
Cleaning The Substrate
River rocks look beautiful though they are harder to clean. Their large size leaves much space between rocks for waste to settle.
Smaller gravel and sand are easier to clean because it’s compact. There isn’t much space for the waste to accumulate.
Delicate Fish
Aquarists should also consider their fish’s activity and the fragility of their body.
Fish that like to dig, like cory cats, should have small substrates that won’t harm their bodies. Sand, substrates for plants, and smaller gravel all work well.
Another example is loaches. Loaches have very delicate skin because they are “scaleless” fish. Their bodies suffer injury much more easily than other fish. Sand or substrates for plants are usually best.
Other species, like some cichlids, like to move the sand around to create a landscape that suits them. Larger substrates make this more difficult or impossible and can cause injury.
Fish That Eat Substrates
Whether a fish is prone to eating substrate is another important consideration. Certain species like to eat the substrate, which can cause impaction.
Goldfish are a prime example. They are scavengers that constantly search for food. They won’t hesitate to eat gravel small enough to fit in their mouths. Larger gravel and rocks are better suited for them.
Color & Appearance
Color and appearance are something aquarists should consider last. It’s more important to determine what is best for the fish first.
Colorful Substrate
It’s easier to pick a colorful substrate when keeping a non-planted tank of mid-dwelling fish.
Aquarists can generally pick whichever substrate suits them best in these cases.
Limitations
Cichlids and marine fish owners are more limited on color and appearance.
Most cichlids like to dig and can create a commotion with hard gravel or pebbles. They also need higher pH and alkalinity levels than other freshwater fish.
Marine fish also need high levels of pH and alkalinity.
Cichlid and marine fish owners usually stick with crushed coral or aragonite sand.
These materials usually come in white, off-white, or tan colors. There isn’t much variety to choose from.
Smooth aragonite sand creates a finer look than coarse crushed coral. There is some say in the aesthetic the substrate creates in the tank.
Making The Space Look Smaller Or More Open
The color of the substrate has a significant impact on the look of the aquarium.
Darker substrates tend to make aquariums look smaller than they are.
Many hobbyists with smaller tanks prefer white substrates. White makes tanks look larger and more open.
Brown is another popular choice for aquarists with less time for cleaning. A brown substrate is good for hiding fish and food waste between cleanings. White makes every bit of waste obvious.
How To Clean Substrate
Cleaning substrate is a fairly easy task for those with a gravel vacuum.
A gravel vacuum comprises a long hard tube that cleans the gravel. It also has a cap and a thin, flexible tube that removes the water.
The structure of the gravel vacuum makes it simple to remove waste from between pieces of gravel. The gravel is too heavy and big to get sucked through the tubing; it simply falls back down after cleaning.
Cleaning Sand & Light Substrate
Cleaning sand and lighter, smaller substrates is slightly trickier.
Don’t stick the gravel vacuum straight into the substrate; it sucks the substrate right out of the tank.
Instead, hover the gravel vacuum about 1 in (2.5 cm) over the surface of the substrate.
Gently sweep the vacuum over the substrate to create a current. The motion encourages any waste to lift from the substrate, and the vacuum sucks it up.
How Often You Need To Clean The Substrate
We recommend cleaning the substrate once every week to two weeks.
How often the substrate needs cleaning depends on the bioload of the aquarium.
Tanks with more fish need cleaning more often.
Those with dirty fish, like goldfish and plecos, need more frequent cleaning.
Aquariums with only a few fish need less cleaning, especially if those fish don’t create a lot of waste.
