Topsoil Aquarium: Dirted Tank Setup, Pros, and Cons

Can you plant aquatic plants in dirt?

Of course, you can! So many bottoms of rivers, swamps, and lakes are nothing but wet soil, more commonly known as mud. Plants love it, but they aren’t the only living things that thrive on dirt.

The real question is whether you can use topsoil for your aquarium without it looking and smelling like a swamp. And without killing your fish, preferably.

When done properly, organic soil is one of the best choices for planted aquariums.

Topsoil aquarium setups require little investment, as they utilize other inert substrates like gravel or sand to “seal” the dirt. This way, plant roots can reach the nutrient-rich soil, but the water column stays clean and there are no ammonia leakages to threaten your livestock.

Dirt, gravel, and sand are considerably cheaper than commercially produced enriched aquarium soils. They can be obtained for free even. Before the invention of specialized soils for planted tanks, dirt was the only active substrate available. Its accessibility allows for freer experimentation, but soils not meant for aquariums should be handled with a certain degree of care.

Let’s explore the dirty little aquarium secrets!

What Is Topsoil

Topsoil refers to the upper layer of the Earth. Some classifications include the very surface, but most models consider the first couple of centimeters of organic matter and humus, with the topsoil starting just below that.

In any case, topsoil is a nutrient-rich environment that supports the life of virtually all terrestrial plants. Water plants are not that different from their dry-land cousins. Indeed, on average, aquatic greenery absorbs a lot of nutrients from the water column, but the so-called root feeders can develop powerful root systems if it suits them.

The availability of nutrients in the substrate is exactly what stimulates root development.

But can you use any kind of topsoil in a fish tank?

Top Topsoil Types for Tanks

There are certain types of soil that are suitable for aquariums. Some work better due to their higher concentration of nutrients. You can buy organic potting soil or scoop some dirt from your backyard.

Collecting dirt from the forest would also work.

These are the type of soils and their characteristics that work well for planted aquariums:

  • Organic soil, i.e. no artificial fertilizers or other form of chemical treatment for pest control. These things save plants but kill fish.
  • Organic potting soil often contains peat moss which is excellent for absorbing and storing nutrients. It’s so good that commercial aqua soils also use it.
  • More dirt than hard particles — A relatively low “debris” count allows for easier and safer work. Small branches and other organics that have not fully turned to dirt aren’t the end of the world, but the decomposition process releases ammonia. You’ll always have to cap the dirt well with sand (or fine gravel) to prevent ammonia leakages, but larger particles also make it harder to smooth things out. Again, some branch particles are par for the course but try to get a good “pure” dirt ratio.
  • Compost — Fully cooled-down compost can work wonders with plants. My compost-based tank is thriving, but it is releasing a high quantity of tannins. I love the water color and it certainly makes sense. I sifted the compost to very fine dust but a good 30% of it was formed from timber and trimmed tree branches.

Tannins are relatively easy to remove but I am leaving my tank on the brackish side.

Topsoils to Avoid

If you are planning a purely planted tank, using topsoils that have not been chemically treated remains important, albeit a bit less so.

But if you want to keep fish or shrimp, don’t use soil mixes that have been treated with pesticide or herbicide.

Another thing to avoid is perlite. This volcanic glass helps terrestrial plants grow but will poison your swimming pets.

Topsoil vs. Aqua Soil: Differences and Advantages

Not so ago the only soil for aquariums was found in gardens and gardening shop sections. Today, experienced hobbyists use specialized aquarium soils to great effect to grow captivating aquascapes.

Commercially produced aqua soils like Fluval or Tropica have a lot in common with topsoil:

  • Both topsoil and commercial aqua soils support lush tropical fish tanks
  • Both release ammonia
  • Both types of sols require additional maintenance in the first weeks
  • Aqua soils, just like topsoil, require experience in the hobby

Active substrates like soil add a powerful agent to your fish tank. They must be understood and properly integrated into your ecosystem. Inert substrates like sand require considerably less maintenance and will not surprise you with a sudden spike in ammonia or nitrates.

There are some major differences too, though. 

  • Quality topsoil supports plants longer than aqua soils
  • Conversely, aqua soils produce consistent results
  • Once you get to know a brand, you know what to expect and how to deal with problems
  • The latter is true to a large degree for commercially sold organic gardening soil mixtures

Of course, organic potting soil will cost less than aqua soil, and dirt from the forest or the backyard is free. Even if you include the inert substrate, the bottom of your tank will remain a relatively minor investment.

What’s more, inert substrates like gravel have greater visual variety than aqua soils. Overall, potting soil is a more affordable way to get your dirt tank going and to acquire some experience.

Can You Use Uncapped Soil in a Fish Tank?

Soil by itself is dangerous in a fish tank. It will make the water column cloudy and is likely to release too much ammonia. Plants could tolerate this and over time an ecosystem could emerge. After months the conditions may change sufficiently to support complex life forms like shrimp or snails, and even fish.

But it is much safer, quicker, and more visually appealing to set up a dirted aquarium properly.

Dirted Tank Setup

There are two ways to set up a dirted aquarium: you either cover the entire bottom or you don’t.

In both cases, you must cover the layer of dirt with a thick layer of sand or gravel. Sand is the better option because its weight distributes much more evenly and won’t press the dirt into undesired craters. Still, a sufficiently thick layer of small gravel works fine.

Regardless of how much soil you want to use, sifting through it is a good way to remove larger particles. A high count of organic matter, particularly wood, can make your water look brackish due to tannin release.

Another thing to consider with commercial potting soils is that they may contain manure, chicken, or otherwise. It is not bad for the plants, but it should be present in moderation in the potting mix.

Too much organic fertilizer can lead to strong ammonia spikes that can be fatal for fish.

If your potting soil contains manure, monitor regularly the tank ammonia levels for a couple of months. Regular water changes and some additional water circulation will likely exhaust the main ammonia release to make it safer for livestock.

Setting up a dirted aquarium requires some patience.

Fully Covered Bottom

Here’s how you can set up a tank with a bottom fully covered in dirt:

  1. Evenly spread 2 centimeters or 1 inch of dirt across the dry aquarium. Its thickness will grow as it grows shorter.
  2. Pack it with a hand.
  3. Sprinkle with water and let it compact by itself for 30 minutes.
  4. Sprinkle a bit more and pack the dirt with your hands. Kitchen gloves help the mud stay at the bottom of the tank instead of sticking to your hands.
  5. Let it rest for 30 minutes.
  6. Add 4-6 centimeters or 2-3 inches of sand or small gravel evenly across the dirt, to cap it off.
  7. Add a few inches of water. Clean whatever debris flows up with a gravel siphon.

But you may prefer to cover only part of the aquarium with dirt. Covering the corners of the fish tank with soil often is visually less impressive than sand, gravel, or even pebbles. Substrate color affects fish color quite a lot; generally, darker substrates make colors stand out more.

Partially Covered Dirted Tank

Alternatively, you could create a couple of dirt mounds where root plants will establish. All other plants will benefit from the nutrients released in the water column.

To cover part of the fish tank bottom with dirt:

  1. Create a mound of dirt, thick an inch or more. It can cover a spot or two of the aquarium. To avoid spillage, encircle the parts for dirt with sand or gravel. If you want to cover 85% of the fish tank with dirt, lay the inert substrate around the edges of the tank in advance.
  2. Compact the soil. Use kitchen gloves.
  3. Sprinkle slowly with water.
  4. Wait 30 minutes.
  5. Pack the mud with your hands. The goal is to expulse any air pockets and to shape it the way you want. Sprinkle more water when necessary. 
  6. Let it rest for 1-24 hours.
  7. Pack it some more.
  8. Cover everything with 4-6 centimeters of sand or gravel. Or both, and some pebbles on top.
  9. Add a few inches of water. Clean whatever debris flows up with a gravel siphon.
  10. Send photos.

You can experiment with the soil surface. Less soil also makes the water column easier to maintain early on. It may support relatively fewer plants for shorter but a balanced ecosystem may be more attainable.

Pros and Cons of Topsoil Aquariums

Let’s cap off our topsoil exploration with an at-a-glance summary of the advantages dirt brings to the aquarium:

  • Topsoil contains plenty of nutrients for plants to thrive
  • Garden soil is very affordable or even free
  • It works great with beautiful inert substrates like sand or gravel

These are major perks of topsoil, but it does come with some considerations. Here are the cons of topsoil:

  • Could leach ammonia if poorly capped
  • Requires frequent water changes early on

As long as the soil is well-covered in sand or gravel, ammonia spikes should become a non-issue. Depending on the soil you have, you could end up with a higher tannin concentration than you want, but tannins are great for the fish’s immune system, so there’s that.

Peat moss is one of the best additions to an aquarium soil mixture, as it conducts a powerful nutrient exchange.

Overall, using topsoil in an aquarium is a great low-tech way to start a lush planted tank. Cap if off well with an inert substrate and conduct regular water changes the first 4-6 weeks. You can plant your aquatic plants in the dirt pretty much straight away. They won’t need any fertilizers to thrive.

Once the nitrogen cycle is going strong and the ammonia levels are consistently low, introduce the first live inhabitants.

Enjoy your dirted tank!

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