How Much Gravel For a 20-Gallon Fish Tank?

Fish tanks are a great way to decorate your home or office. They come in many different shapes and sizes, and with a little creativity, you can really make them pop! One of the most important aspects of any fish tank is the aquarium substrate that you use. Gravel plays an integral role in giving your fish tank its unique look. It also provides a comfortable place for plants to grow, which provides oxygen for your aquatic friends. It is important to know how much gravel is needed for each tank size and when it should be added so that you can have everything set up before adding water! So, How Much Gravel For a 20-Gallon Fish Tank?

A 20-gallon fish tank would need about 25lbs of the substrate (which equals 50 lbs for the 100 and 150). To allow room for plants, it’s best not to fill the entire bottom with gravel. The rule of thumb is to have about an inch of gravel for every gallon of water.

This article will discuss how much gravel you need to set up a 20-gallon tank and go over the things you need to consider. Then, I will guide you through the different types of gravel and decide which one will be best for your fish tank.

How Much Gravel For a 20-Gallon Fish Tank?

A 20-gallon fish tank would need about 25lbs of the substrate (which equals 50 lbs for the 100 and 150).

The substrate depth should range from about one inch up to three inches, depending on the type and needs of your fish.

If you have live plants in the tank (either rooted plants or floating), it would be wise to get at least an inch deeper for gravel than recommended for a tank without plants.

For the most part, what kind of gravel you use doesn’t matter as long as it’s not sharp and can be filtered out easily with some water changes.

It would also make sense to look at your tank type/size when choosing substrate depth to get something appropriate for the size of your tank.

If you have a larger, deeper tank with fish that like to dig and create tunnels, then three inches is probably the best depth for gravel. If it’s not deep enough, they may get stuck in some areas and starve due to lack of food.

On the other hand, if it’s too deep (over three inches), some fish can’t get to a surface or may not find food. Before adding gravel, you should make sure that the tank is clean and free of debris to add extra dirt.

You will also want to let water sit in the tank for at least 24 hours before adding gravel to the tank for it to sterilize properly.

Standard Gravel Calculation Procedure For 20 Gallon Tank

When filling your fish tank for the first time, it is important to know how much gravel you need. The most accurate way of determining this is by using a conversion calculator.

This is a simple calculation to find out how much you need for your 20-gallon tank.

20 gallons X 12 inches (standard height) = 240 cubic inches of space in the bottom layer of gravel

240/1000= .24 feet or 24″. Add this number to the depth that the manufacturer on the substrate packet recommends.

The result will be the total amount of aquarium stones needed for a 20-gallon fish tank.

1 lb per gallon of water is the traditional approach to estimate how much gravel to put in your fish tank. This method will give you a depth of 1-2 inches which will be sufficient to outset the biological filtration.

But, if you are a perfectionist and want to measure the exact amount of gravel, there is a standard calculation procedure. Here is a demo of the step-by-step process of the calculation of a 20-Gallon high tank that has a length of 24’’, width of 12’’ and height of 16”:

  1. First, multiply the length of the tank by the width of the tank: 24” X 12”= 288”
  2. Divide the answer by 10: 288”/ 10 = 28.8
  3. Then, convert your answer into kilograms; dividing it by 2.2: 28.8 / 2.2 = 13.09 kg (28.6 pounds)

Therefore, 13.09 kg (28.6 lbs) is the amount of gravel needed for your 20 Gallon Long fish tank.

You can use the same formula for any other type or size of the tank.

When Should I Put Gravel In My 20-Gallon Tank?

If you are a beginner, it is best to put gravel in your fish tank after adding water. Your 20-gallon tank should be at least half full before putting any substrate inside of it.

After filling the excess space with water and letting it settle for one hour, add your desired amount of aquarium stones (or clay pebbles) and place the gravel on top.

If you are an experienced fish owner, it is best to put in your substrate before adding water to any tank size (even a 20-gallon).

The reasoning for this is that oftentimes some of the sand or stones will fall into the cracks between rocks or inside crevices where they are nearly impossible to remove.

After filling the excess space with water and letting it settle for one hour, add your desired amount of aquarium stones (or clay pebbles) and place the gravel on top.

Can I put Gravel in a Fish Tank Before Adding Water?

Yes, make sure not to stir up the gravel too much and remove any rocks or sharp objects before adding water to avoid injury to your fish.

You do not need to add water before you can fill your tank with gravel. However, if you have a sand substrate or plan on having one, this will take some time to settle, and avoid putting large amounts of it into tanks with fast currents. You may want to add a little bit of water first so that the sand will not sink.

Can I put Gravel in a Fish Tank After Adding Water?

Yes, you can also fill your tank with gravel after adding water, but it is important to make sure there are no sharp edges or rocks on the pieces as they may cause injury and/or damage to the fish as well as the tank itself.

You can choose to do so if you want but be warned that what is submerged will be soaked with the water, which may lead to some sogginess if you’re not careful.

How To Choose The Right Kind/Depth of Substrate?

Choose an appropriate substrate for your needs and what you’re looking for in appearance, plant life, etc. Next, you need to make sure it has no sharp edges or rocks. These may cause injury to your fish. You can do this by checking the water before adding anything in and stirring it around to ensure no sharp edges or rocks.

Needs Of Your Fish

The needs of your fish will determine what kind of substrate you need. For example, if you have a goldfish that loves to dig and make tunnels in the sand, then placing gravel with rounded edges might be better for them than sharp-edged pebbles or rocks.

Plants You Use

If you want plants in your tank, you need to ensure that the substrate has enough room for them. This is especially important if your plants have roots to grow and get a steady source of nutrients.

Your Tank Type/Size

The type or size of the tank will determine the best kind of gravel. For example, if you have a heavily planted 20-gallon tank and want a lot of gravel in it because they give more depth.

How Often Should You Clean Gravel In A Fish Tank?

It would help if you cleaned the substrate every few weeks, depending on how many fish live in your tank and any buildup. You can clean it with a gravel vacuum that has an attachment for picking up the substrate, or you could use your hands and scoop it out.

How Often Should You Change Gravel In A Fish Tank?

It would help if you changed the gravel in your tank every few months to ensure that no harmful bacteria are growing on it. If there is any buildup, your water may not be as clean or healthy for the fish. You can change it by using a gravel vacuum to remove all of the sand and pebbles from the bottom so that you’re left with just an empty tank.


In conclusion, the best way to determine the amount of substrate is by calculating how many pounds you need for your tank. When caring for their fish, a common misconception is that they need to fill up the tank with gravel, sand, or something else. But in the case of a 20-gallon fish tank, you only really need about an inch’s worth of substrate (or less).