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The topic of how much gravel or substrate you should put in a fish tank is actually far more complex and important than many new aquarium owners realize, and also quite a few experienced ones as well. More accurately, we are talking about the substrate, because there’s not just gravel. There are quite a few different materials that can be used, some better than others.

When filling an aquarium with gravel, you should use 1 lb of substrate per gallon, this allows plenty of space for beneficial bacteria to grow. 

It’s not just a case of asking how deep the gravel should be in a fish tank, it’s about understanding the type of tank you have, the type of fish, what you have in the tank with the fish, and also which substrate is the safest, and most flexible.

In this easy guide, we’re going to cover all of those topics, so that you can make an informed decision on the amount of gravel, or another type of substrate, to use in your tank for maximum effect and safety.

Why Use A Substrate In A Fish Tank?

Using an adequate substrate in a fish tank is very important. It’s not just about aesthetics. The gravel, or other substance, that sits at the bottom of the tank plays an important part in other aspects of maintaining a healthy aquarium.

So it’s not just about how deep it is, although that is obviously a factor, depending on the exact needs you are trying to establish.

The type of fish you own will help you to determine what type of gravel you will need for your tank. Some fish prefer finer gravel, while others flourish better with coarse gravel. Some fish like to forage on the bottom of the tank, or dig into it, which will obviously be more difficult with a heavier type of gravel.

Another factor to consider is what else will be in the tank. Plants need certain types of substrate, and a firm base for aquarium decorations is another thing that may have to be taken into account.

Using Gravel For A Fish-Only Aquarium

Some types of fish only require a base that serves the most basic purpose in a fish-only aquarium, meaning that its purpose is just to provide a beneficial bacteria bottom for the tank.

At its most basic, any substrate has one key function. That is for it to allow bacteria to grow and build colonies within it. Building and maintaining a balance of healthy bacteria in the tank and water will really help your fish to thrive.

A second reason, the second most basic practical reason for using gravel in the bottom of the tank, is that it gives orientation to the fish. If there is no layer on the bottom of the tank, the glass and reflections can disorientate them, causing stress.

If you are purely using gravel for these most basic purposes, then a depth of around 1-2 inches will be adequate. If you have bottom feeders within the tank, then you should go slightly deeper than that, and also make sure that the substrate use is quite fine.

Even in a basic setup, the coarseness of the gravel, or other substrates, is important. If you don’t keep on top of stopping food particles from sinking to the bottom of the tank, over time food will rot within the substrate. This can produce hydrogen sulphite, which is highly toxic to fish.

How Much Substrate Will Depend On Plants & Decorations

Another major consideration when trying to define how much gravel you should put into a fish tank is to consider what will be in the tank with fish.

The two most obvious factors to consider are aquarium plants, rocks, driftwood, and other decorations to create a visually attractive, and stimulating environment, as well as provide security for the fish.

If you intend to keep plants in trays, or they don’t have root systems, then the amount of substrate used is irrelevant. This is because they will get most of their nutrients from the water, so don’t need a strong base.

However, plants with root systems will need a finer type of gravel or other substrates in order to grow and remain stable in the aquarium. Not only will it have to be finer gravel, but it will need to be deeper and more compact.

Obviously, the depth and compaction will depend on the size of the root systems. With small plants, you can still get away with less than 2 inches, but for larger plants, you may have to use 3 inches of aquasoil, or soil topped with gravel.

The other part of the consideration is stability for things like large rocks, heavy driftwood, and ornaments that you put in the tank to create a fun environment for your fish.

These can move and fall over if they sink into the substrate. This is not only annoying, but it could be dangerous to the fish.

Types Of Fish Tank Substrate

So now we’ve covered the main consideration is around how much gravel you should use in a fish tank, and we’ve established that up to 3 inches may be needed, depending on what your plans are, let’s look at the different types of gravel, and other substrates you can use in an aquarium.

The choice and selection will depend on the factors we’ve already discussed, plus whether your fish tank is populated by freshwater or saltwater species.

You will also need to consider whether you can layer different types of substrate to ensure the absolute best base for your tank.

1. Aquasoil

Aquasoil consists of up rounded grains that facilitate the flow of water in your aquarium. They work similarly to water filters. There are many types of aquasoils, making it easy to find the perfect match for your tank. Aquasoil is an ideal substrate for aquariums that house aquatic plants.

Being the ideal substrate for planted tanks, it provides beneficial nutrients to your plants without altering the water chemistry too significantly. It will be more pH neutral because high pH can be bad for aquatic plants. The substrate also gives plants a place to anchor in and get settled into the tank, spreading their roots and growing larger in a stable position.

2. Crushed coral and Aragonite

If you intend to have plants in your tank then a substrate that is able to store nutrients for plants is essential. This is because obviously, plants get the majority of their nutrients from the root system.

Aragonite is basically dead coral. Otherwise known as calcium carbonate or limestone rock. Some aquarium base materials sold as sand are actually calcium carbonate. The way to tell this is to test it by pouring a small amount of vinegar onto it. If it starts to froth, then it’s not sand, but calcium carbonate. This is not a problem, as long as it’s well manufactured and has the consistency you need.

However, aragonite and crushed coral is can increase the acidity of the water, sometimes pushing the pH level up over 8.0, so it’s important to monitor the pH levels of the water if you use these.

Crushed coral is slightly different in that it usually consists of larger pieces. This means it can trap things within it more easily. So if food dropping to the bottom of the tank is a problem, then crushed coral could prove problematic.

3. Sand

The thing is, sand is not all the same. As you’ve just seen, some sand is actually crushed coral, dead coral known as calcium carbonate.

You also have beach sand. A lot of people think that that is perfectly acceptable, and free because saltwater fish live near it all the time. But the thing is, beach sand can contain a lot of living organisms, that will subsequently die very quickly once put in the tank, creating a lot of problems for the quality of the water.

But there are two types of sand that are very good for forming the base layer of the fish tank.

The first type of sand is sandblasting sand. This is a coarse type of sand, mostly consisting of quartz. However, it is not sterile, and that needs to be factored in when putting it in the tank.

The second type of sand is play sand. This is fine, soft sand that has been sterilized for use in children’s play areas. So for several reasons, it’s ideal for fish tanks as well.

Sand, whether it’s calcium carbonate, sandblasting sand, or play sand, can form an ideal base because the wet sand creates a natural environment for both freshwater and saltwater species. It forms a great base because it will compact when wet, which will stop food from getting trapped in it, but still allow bacteria to flourish.

4. Gravel

As we initially talked about how much gravel is for a fish tank, let’s talk about actual gravel as the base layer for a fish tank.

Gravel tends to be larger pieces, so very porous as a base. But because it’s heavy, it can form a good base that doesn’t move. It’s also probably the most common base for aquariums because it’s deep to produce, flexible, and sold widely.

It can be bought in different grades, from the course through to very fine, and it’s commonplace to layer different grades. It’s usually quartz, in different sizes. Aquarium gravel is safe to use and does not leak anything into the water, making it neutral.

However, it’s very porous, which means food and excrement can sink into it and rot. At the very least, it will need flushing out when you change the water or vacuum. A halfway house is to use it as the top layer with a less porous layer underneath, making it a great combination system.

Whichever type of base you use, depth in the range of 1 inch through to 3 inches is the most usual. Sand can be ideal, and with a good filtration system, any type of base is easier to maintain.

So the key consideration of how much gravel for fish tanks that thrive, is one that meets the needs of the fish, and the plants, and allows easy maintenance of hygiene within the tank, while using the minimum depth you can get away with, usually in the range of 2-3 inches.


Hi, my name is Jordan. I've been in the fishkeeping hobby since my childhood. Welcome to my blog where I help fishkeepers enjoy the hobby by offering free guides, advice, & product reviews. Read more...