2-inches of dry sand is a sufficient cap for a dirted aquarium. This is thick enough to prevent too much ammonia from leeching into the water column. It will also create an anoxic zone on the bottom layer to promote the growth of anaerobic denitrifying bacteria.
In this article, we will share our exact process for capping a dirted aquarium with sand.
Prepare your dirt and sand
How much dirt do you need?
You can easily calculate how much soil you will need if you know the dimensions of your aquarium’s footprint. Our 20-gallon tank is 12-inches wide and 24-inches long.
We multiply those numbers together and get 288. We then multiply that by the depth we want, which is just 1 inch. So we need 288 cubic inches of soil for this tank.
Most bagged soils will display the volume in litres. To convert our 288 cubic inches we would divide by 61, which gives us 4.7 litres of soil. The bag we bought was 20L, so there was plenty left over.
This is how you would calculate your required dirt volume if you’re only using dirt. Many dirted tank keepers will prepare their soil with additives and other organics, such as peat moss and manure.
In this case, your total dirt base layer volume remains the same, but you may have less soil because you will replace a portion of it with the peat moss and manure.
Soak your dirt in water
It’s beneficial to soak your dirt in water before adding it into your tank. This will help to reduce large pockets of air from being trapped inside.
What we do is measure out our dirt into a 5-gallon pail and add water to that. Our goal is to create a muddy consistency, not a soupy one. You should be able to mould your dirt into loose shapes.
What we do is fill another 5-gallon pail with water and dechlorinator. Then we add small amounts of water to the pail with dirt, mix it around, and then repeat until we have the consistency we want.
If you added too much water you can scoop some of the dirt out into a fish net and strain it over your bathtub. Repeat this until you’ve removed any excess water.
Add a sand border around your aquarium perimeter
We like adding a border of sand on the bottom of our dirted tanks to prevent the dirt from leeching into the water column. The sand border keeps the dirt away from the edges of the tank where it’s more vulnerable to escape.
What type of sand can you use?
Pretty much any type of sand is safe to use for aquariums, as long as it doesn’t contain any additives. Pool filter sand is a popular and inexpensive option that many fishkeepers use.
We chose to use the Super Naturals brand of aquarium sand:
Add 1 inch of dirt
We then add a 1-inch layer of the dirt to the bottom of our aquarium by scooping it out of our 5-gallon pail. We contained the dirt inside the sand border we created earlier.
Add at least 2 inches of dry sand cap
Next, cover your dirt layer with at least 2-inches of dry sand.
Why is a cap important?
The main purpose of a sand cap is to create an anoxic zone at the bottom of your tank where there is no oxygen. This will allow anaerobic denitrifying bacteria to cultivate, which will remove nitrate from your tank.
Oxygen will penetrate about 2-inches through a sand substrate, but not much further than that. This means that the bottom layer of dirt will contain little or no oxygen.
The sand cap also prevents high concentrations of ammonia in your soil from leeching into the water column. Although you will likely experience ammonia spikes in the first couple weeks regardless. But this will help you cycle your dirted tank more quickly.
Use a plate, collander or plastic bag to fill water
Once your dirt and sand cap are in place, it’s time to begin filling your tank with water.
Simply pouring water directly on top of your sand will create a crater and may allow the dirt beneath to escape into the water column. This will create an incredible mess that will take a long time to clarify and will cause a huge ammonia spike.
Instead, place a collander, plate or plastic bag on top of your sand before pouring water into it. These objects will receive most of the impact and will distribute the water across a greater surface area.
In other words, it will prevent the substrate from being disturbed too much.
How thick should your sand cap be?
A sand cap of at least 2-inches is important. This will be sufficient for creating an anoxic zone at the bottom of your tank.
The anoxic zone does not need to be thick to be effective. And if you have a small-mid sized aquarium then you may want to maximize the space for your aquascape and fish.
Therefore, we recommend no less than a 2-inch cap with a 1-inch base layer.
A deeper substrate is more beneficial. There is essentially no upper limit to how deep your substrate should be, as long as your cap is at least 2-inches.
Can plants grow in aquarium sand?
Aquatic plants grow well in aquarium sand as long as they have access to adequate fertilization. Heavy root-feeding plants will likely require a nutrient rich substrate beneath a sand layer in order to remain healthy.
Many fishkeepers believe that sand is too fine to allow roots to grow successfully. This is not the case at all.
We, and many other fishkeepers, have great success growing plants in sand. In fact. we’ve had more success growing plants in sand than in inert gravel.
Plants have a variety of macro and micronutrients that they need to support healthy growth and development. Planting in an inert substrate with no fertilization will not provide the plants with adequate nutrition – especially root-feeding plants.
So if you want to use sand, gravel or another inert substrate in your planted freshwater aquarium, we recommend adding an active dirt or aquasoil substrate beneath it so the roots can feed.
Can you use gravel instead of sand?
You can absolutely use gravel as a cap instead ofo sand. In fact, we’ve seen evidence from other fishkeepers that gravel can be equally, if not more, effective than sand at producing anaerobic bacteria.
However, we have not tested gravel ourselves so we cannot confidently recommend it. It’s possible that a gravel cap would require more than 2-inches because it’s more corase, and may allow oxygen to penetrate deepr than with sand.