Can You Use Vermicast for an Aquarium?

There are many different types of aquarium substrates. To make the best decision for your fish, you must understand how it affects the entire habitat. So, is vermicast a good choice for your tank?

You can use vermicast for some freshwater aquariums, but the level of nitrates is too high for saltwater tanks. Vermicast is terrific for keeping plants healthy, but its effect on water parameters may not be conducive for all fish.

Read on to learn about the benefits and considerations of vermicast, as well as the steps to use vermicast for an aquarium.

How Vermicast Supports Healthy Plant Growth

Aquarium plants, often called submerged grown plants, depend entirely on substrates for nutrients. Without access to oxygen or nutrients to absorb through their leaves, these plants can only access nutrients through their roots. Therefore, having a substrate that can fulfill all of the plant’s nutrient needs is crucial.

Like plants above the ground, aquarium plants cannot survive on vermicast alone, which can cause nutrient toxicity. Instead, vermicast should be mixed with another substrate, most commonly organic soil.

Understand How Vermicast Affects Water Parameters

Water parameters are values for many different elements of aquariums, such as water hardness, pH, tannins, and nitrates.

Before using vermicast in your aquarium, you must understand how it affects each parameter:

Water Hardness

One important water parameter in aquariums is general water hardness – or GH. Essentially, GH measures the number of minerals in the water, primarily calcium and magnesium.

The target water hardness for your aquarium depends on the type of fish you are housing. For example, fish from South America usually require soft water, while African fish such as African Cichlids thrive in harder water. 

Vermicast has magnesium and calcium, so it contributes to water hardness. This in itself is not good or bad.

Instead, vermicast should be part of an ecosystem that can maintain the appropriate water hardness for your fish. If your fish need soft water, you may need to omit vermicast from your tank to keep the aquarium balanced.

Sudden changes in water hardness can be just as harmful to your fish as unhealthy water hardness. Whenever you make changes to your tank, you must add them slowly to allow the fish and plants to adjust gradually.

Adding pieces of coral to the tank is one popular method of increasing general water hardness. You should not use this approach when you have vermicast in an aquarium. Coral is very sensitive to nitrates, which vermicast is high in. 

Water pH

Like water hardness, the ideal water pH of your aquarium depends on the requirements of your fish. A neutral pH is 7, with lower values being more acidic and higher values more alkaline. Generally, freshwater fish prefer a neutral pH, while saltwater fish tolerate more alkaline water.

Vermicast has a neutral pH of 7, which is in the acceptable range for most fish. If your fish require more acidic or alkaline water, using less vermicast will make it easier to maintain a different pH.   

You should test the pH levels of your aquarium weekly to ensure stability. You can do this with an electronic meter or paper strips. Again, if you need to alter the pH in your tank, you should make these changes slowly to avoid shocking the fish.


Nitrates pose the most serious risk to your aquarium. Vermicast contains nitrates, an ionized form of salt that occurs naturally in animal waste. Because vermicast is worm waste, there is no way to avoid this.

However, if you add vermicast to your aquarium, you should not prepare it with animal waste. Worms fed only kitchen scraps will produce vermicast with fewer nitrates.

Substrates are not the only contributors to aquarium nitrates. Fish food and fish waste also produce nitrates. 

Plants can use nitrates for food, so having an entirely nitrate-free tank is not necessary. However, nitrates should be very sparse if you add coral to your tank. Coral is particularly sensitive to nitrates and will die quickly with significant exposure.

Most freshwater tanks cannot tolerate a nitrate level above 40ppm, and planted tanks have an even lower threshold of 30ppm. Saltwater tanks also have a limit of 30ppm of nitrates. Brackish and pond water, however, can withstand high levels of nitrates.

Fish exposed to too many nitrates will often:

  • Become sick
  • Have lower reproduction
  • Lose bright coloring
  • Have stunted growth
  • Die prematurely


Ammonia is a hazardous aquarium chemical and can quickly kill fish and plants. The good news is that the redworms that create vermicast are also very sensitive to ammonia. Since these worms cannot process material with ammonia, the castings they produce contain little to no ammonia.

After excretion, vermicompost will continue to release small amounts of ammonia until it has decomposed completely. Fortunately, the process of making vermicast eliminates all of the excess decomposed material found in vermicompost. 

However, you should be aware that the terms vermicompost and vermicast are often used interchangeably, so you need to confirm you are using pure castings to ensure you are not adding additional ammonia to your tank.

When it comes to ammonia, fully decomposed vermicast is a safe choice for an aquarium substrate.

How to Use Vermicast for an Aquarium

Setting up an aquarium with vermicast is similar to any other substrate. However, it will require an additional layer of substrate for capping.

To use vermicast for an aquarium, you should use the following steps:

  1. Add the vermicast mixture.
  2. Cap the vermicast.
  3. Slowly add water.
  4. Add equipment.
  5. Add plants.
  6. Introduce fish.

I’ll break down each of these steps in the sections below.

1. Add the Vermicast Mixture

Vermicast should be the very bottom layer in your aquarium. It can be a thin layer or mixed into another material. The most common mixture is vermicast and organic soil.

Your best option is to purchase a soil substrate made specifically for aquariums to avoid dangerous chemicals.

When added to soil, vermicast should not exceed 20% of the mixture. Initially, blending 10% vermicast or less is best to see how your tank fares. If there are no adverse effects, you can increase the ratio if you would like.

Before adding the vermicast to the tank, use a colander to separate the smallest pieces.

The soil mixture should not be more than 1 inch (2.5 cm) thick since you will also need to cap it.

2. Cap the Vermicast

While vermicast is good for the roots of plants, your fish should never come into direct contact with it. Instead, you should add an additional substrate layer to cover or ‘cap’ the vermicast.

It is best to use pebbles or gravel for capping. A layer of 0.5-1 inch (1.25-2.5cm) should be sufficient, ideally not exceeding 1.5 inches (4 cm) in combination with the vermicast mixture.

Like the vermicast mixture, you should sift out the smallest, dust-like particles before adding the capping.

3. Slowly Add Water

Even if you strain your substrates well, there will still inevitably be small particles in the substrates that will come to the surface when water is added. A few particles are expected, but if your water gets very cloudy, you do not have enough capping to keep the vermicast in place. 

To make this process easier, start with only a few inches (5+ cm) at a time and add water slowly. After the tank is full, you should test the pH and water hardness levels to ensure it is safe to add your plants.

4. Add Equipment

At this stage, you should add any equipment you would like to use, such as filters, heaters, and bubblers. To do this, follow the instructions for your unique device. It is a good idea to test the equipment before you proceed so you can address any issues before you get too far along.

5. Add Plants

Once your substrates have settled, and your water is clear, it is time to introduce plants into the aquarium. Gently press the plant roots into the substrate using a stick or your hand. You must ensure that the roots reach the soil mixture and are not stuck in the capping so they can access the nutrients.

6. Introduce Fish

An aquarium should be well established before adding any fish. Before adding fish, you should wait a minimum of two months after adding your substrate and plants. This allows time for the water levels to stabilize, healthy bacteria to grow, and the plants to become established.

After a final pH and water hardness test, you can add your fish.

This video details the process of slowly acclimating fish to their new home:

Final Thoughts

Adding vermicast to your aquarium can help maintain healthy plants. Before adding vermicast to your aquarium, consider the water parameters you will need for your specific habitat.

These parameters are:

  • Water hardness
  • Water pH
  • Nitrates
  • Ammonia

You can follow the steps above to use vermicast for an aquarium.

Alexander Picot

Alexander Picot is the founder of and its lead content writer. He created the website in 2022 as a resource for horticulture lovers and beginners alike, compiling all the gardening tips he discovered over the years. Alex has a passion for caring for plants, turning backyards into feel-good places, and sharing his knowledge with the rest of the world.

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