How much gravel do I need for a 20 gallon high tank?

When setting up a new aquarium, one of the most important components is the substrate or gravel at the bottom. Gravel serves several key functions – it allows beneficial bacteria to colonize and helps break down fish waste, it anchors live plants and decorations, and it impacts the aesthetics and style of the tank. Choosing the right amount of gravel for your tank is important to help create a healthy environment for fish and plants.

Quick Facts on Gravel for a 20 Gallon Tank

Here are some key facts to consider when figuring out gravel needs for a 20 gallon high aquarium:

  • A standard 20 gallon high tank has a base of 24 x 12 inches, providing about 288 square inches of floor space.
  • Most experts recommend a gravel depth of 1-2 inches for freshwater aquariums.
  • At 1 inch of gravel, you would need around 2-4 lbs of gravel for a 20 gallon tank.
  • At 2 inches of gravel, you would need 4-8 lbs of gravel for a 20 gallon tank.
  • Gravel is sold in 5 lb or 10 lb bags usually, so you may need 2 bags for a 1 inch layer or 4 bags for a 2 inch layer.
  • Rinse gravel thoroughly before adding it to the tank to remove dust and debris.

The exact amount will depend on the type of gravel you select and if you slope or contour the substrate. In general, most 20 gallon high tanks work well with around 5-10 pounds of gravel. Read on for more details on calculating the proper gravel amount!

Factors to Consider in Gravel Amount

There are a few key factors that impact how much gravel you need for your 20 gallon aquarium setup:

1. Tank Dimensions

The dimensions of the tank’s base determine the surface area that needs to be covered with gravel. Standard 20 gallon high aquariums have a base of 24 x 12 inches, which equals 288 square inches (24 x 12). This gives a good starting point for estimating gravel needs. Tanks with a larger base area may need more gravel than a tall, narrow tank of the same volume.

2. Gravel Depth

The depth of the gravel layer is arguably the most important factor. Deeper gravel beds provide more space for healthy bacteria colonies to thrive and help anchor plants more firmly. However, gravel also takes up space that could otherwise be used for aquarium water. Recommended gravel depths for freshwater aquariums are usually:

  • 1 inch minimum gravel depth
  • 2 inch gravel depth for planted tanks
  • Up to 3 inches for exceptionally heavily planted set-ups

For most standard 20 gallon high planted aquariums, a gravel depth of 1.5-2 inches is ideal.

3. Type of Gravel

The weight of gravel can vary based on the material. Natural colored gravel is often heavier than artificially colored or coated gravel. Gravel also comes in different grain sizes from fine sands to pebbles over 1 cm in diameter. As a general guide:

  • Fine gravels have a density of about 100 lbs per cubic foot
  • Medium sized gravel about 90 lbs per cubic foot
  • Large pebble gravel around 85 lbs per cubic foot

So you may need a bit more bulk light weight gravel compared to dense, heavy natural gravel to achieve the same depth.

4. Aquascaping Approach

The aquascaping style you want to achieve can also impact gravel needs. For a flat even substrate layer, you can easily calculate from the surface area and depth. But sometimes aquarists contour gravel to create sloped landscapes and high or low areas in the tank. This takes more gravel than a flat layer, so factor this in if taking that approach.

Gravel Amount Estimates

Based on typical recommendations and assuming a standard 24 x 12 inch base 20 gallon high aquarium, you can estimate gravel needs:

1 Inch Gravel Depth

  • 1 inch over 288 square inches would be about 2.4 lbs of gravel assuming medium sized gravel density.
  • Round up to 2-4 lbs of gravel for a 1 inch depth.

2 Inch Gravel Depth

  • 2 inch depth over 288 square inches would equate to about 4.8 lbs.
  • Round up to 4-8 lbs of gravel for a 2 inch depth.

These are rough estimates assuming an evenly distributed flat gravel layer in a standard 20 gallon tank. Actual amounts may vary slightly up or down.

Choosing Gravel Quantity

When selecting gravel quantity, keep these tips in mind:

  • Buy more gravel than you think you need! It’s easy to remove extra, hard to add more later.
  • Aim for the high end of estimates, especially if sloping substrate.
  • For planted tanks, choose gravel estimates based on 2 inch depths.
  • Select gravel sold in 5 lb or 10 lb bags for easier estimating.
  • Rinse gravel thoroughly before placing it in the aquarium.
  • Add gravel slowly and evenly, spread it flat for a level substrate.

For most 20 gallon high tanks, purchasing two 5 lb bags or one 10 lb bag of gravel will provide plenty to work with. This will give you the flexibility to create slopes, piles, or uneven terrain if desired while achieving close to 2 inches of depth across the majority of the tank bottom. Always buy extra because it’s challenging to add more gravel later without completely redoing the tank.

Gravel Placement Tips

Once you have purchased the desired quantity of properly rinsed aquarium gravel, follow these tips for placing it in the tank:

  • Add a little gravel at a time, spread evenly to help create a flat level layer.
  • Aim for between 1-2 inches of depth across the majority of the bottom.
  • You can contour and slope certain areas depending on your aquascape plans.
  • After placing all gravel, use a straight edge to check for even levels if aiming for a flat look.
  • Avoid making the gravel too deep near the front – this can make the tank look smaller.
  • For planted tanks, make sure rooted plants have 2+ inches of substrate.

Take your time placing the gravel and work slowly for best results. Test the depths in several areas with a ruler to ensure your aquascape is turning out as planned. Having extra gravel makes the process easier and allows room for modifications.

Filling the Tank After Adding Gravel

Once your gravel arrangement is complete, you can start carefully adding water to the aquarium. Here are some tips:

  • Place a plate or saucer over the gravel where you pour water to avoid displacing the substrate.
  • Pour water slowly and try not to pour directly onto piles of loose gravel.
  • Use a gravel tidy tool to carefully push any shifted gravel back into place.
  • Add water up to the tank rim, watching for any splashing or overflow.
  • Consider placing a paper towel barrier at the top when finished to catch splashes and drips.

Fill the tank slowly and make adjustments to the gravel as needed. Once full, allow the water to settle everything into place before removing the plate and tidying up the surface.

Adding Extra Gravel Later

While you want to add sufficient gravel in the initial set-up, sometimes you may need to add more later down the road. Here are some tips for correctly adding new gravel:

  • Rinse the new gravel thoroughly before adding.
  • Use a funnel or similar to pour gravel slowly and evenly into tank.
  • Try to avoid dumping gravel directly on top of decor or plants.
  • Use a gravel tool to gently slope and blend the new and old gravel together.
  • Spread out additions across the tank, not just in one area.
  • Do smaller gravel additions over time rather than all at once.

Take care when adding new gravel to an established tank to avoid disturbing the aquascape and hamrful bacteria colonies. Go slowly and make adjustments over days or weeks for best results.


Choosing and placing the correct amount of gravel is an important part of setting up a healthy, visually appealing aquarium. For most standard 20 gallon high freshwater tanks, purchasing around 8-10 pounds of aquarium gravel will provide sufficient substrate when distributed evenly at 1-2 inch depths across the bottom. Consider the gravel type, your aquascaping plans, and equipment placement when calculating amounts. Take your time placing the gravel, fill the tank slowly, and make adjustments to ensure an even, proportional substrate layer. Having extra gravel on hand makes the process easier and allows for modifications down the road. With the right quantity of properly prepared gravel, you will be on your way to creating an ideal tank environment.

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