Can You Use Geodes in Planted Tank?

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There’s no doubt that aquascaping is as much science as it is art. Imagination is your only limit when setting up a unique, vibrant, and breathtaking aquatic environment.

Today, we will discuss planted aquariums and geodes.

Do these go along, and what exactly are geodes? As you’ll see, we have plenty of points to go through.

What Are Geodes?

Simply put, geodes are rocks with crystalized minerals like quartz, gem silica, opal, or rhodochrosite, to name a few materials.

They are some of the most beautiful decorations you could get for your aquarium and offer immense diversity in terms of shape, size, coloring, and overall layout.

Geodes are highly popular among hobbyists as most people collect them for decorative purposes.

You mostly have 2 types of geodes based on the process that formed them:

  1. Volcanic geodes – As the name suggests, these are the result of volcanic activity. Lava contains a variety of minerals like quartz and opal, which are the result of the lava mixing with underground water. The mix creates massive vapors which remain trapped underground, as the lava cannot escape to the surface. The vapor creates empty spaces in the molten rock, which can cool off over time. The result is a crystalized geode, which is a mineralized crystal heart covered by a hard layer of molten lava.
  2. Sedimentary geodes – Sedimentary geodes are the result of time, natural erosion, and pressure. Elements like carbonate materials, opal, or quarts create shapes around voids created by tree branches, shells, or other hard materials. These showcase a lot of variety in shape, content, and size, except they’re generally smaller than volcanic rocks.

Geodes are tricky to process because they’re quite hard. The exterior layer is notoriously difficult to cut through, and it’s a job meant for specialized machinery.

This formidable durability is also the reason why geodes often remain sharp and unpolished, but more on that later.

Are Geodes Safe for Fish?

Yes and no. It depends on the type of geode we’re talking about. Quartz and amethyst are generally safe because they don’t alter the water chemistry.

However, most sedimentary geodes come from limestone and calcareous formations, which means that the geodes in question have high calcium content.

This will seep into the water gradually, increasing the pH over time. This can have potentially devastating consequences because low-pH fish can’t survive in high-pH environments and vice-versa.

So, be very careful which geode you’re getting to prevent dangerous fluctuations in water pH and general parameters. If you don’t want a pH-altering geode, you should skip limestone-based pieces altogether.

But there’s another issue worth discussing. Some geodes are impure and may be contaminated with heavy metals like copper, zinc, or even arsenic.

These are toxic to fish and pretty much all other aquatic lifeforms. The even bigger problem is that even geodes typically considered safe can be muddied by dangerous heavy metal contents.

So, you need to make sure that the geode is safe before adding it to your fish tank. So, let’s discuss that!

Using Geodes in Fish Tank

Given what we’re discussed about geodes so far, it’s understandable why you can’t just buy one and throw it in your tank. Some preparation is necessary to make sure that the geode is safe for your fish and plants.

To achieve that, consider the following options:

  • Selecting the geode type – The type of geode you’re getting makes a huge difference in this sense. As we’ve already discussed, limestone-based geodes will impact the water pH. So, you can only use them in specific setups with fish that don’t mind the higher pH levels. African cichlids come to mind, along with livebearers like guppies, platies, swordtails, etc. This doesn’t mean that limestones are safe for them. These rocks can increase the pH levels beyond what even these fish can withstand.
  • Sharp edges – Many geodes are hard and rough on the surface. It’s not uncommon for them to come with sharp, pointy edges, which can cause severe flesh injuries. Bigger fish or those with larger fins risk experiencing cuts or punctures due to the rocks’ unpolished look. You should always inspect the rock carefully before purchasing it. Otherwise, you will have difficulties making it safe for your fish. The rock is extremely hard, so you can’t really polish its edges that easily.
  • Thorough cleaning – All geodes are processed before the sale. The process involves cutting into the rock to showcase its interior cavity and expose the crystals and minerals. The problem is that the cutting process involves a type of oil that can contaminate the aquarium water. Thorough cleaning and sterilizing are necessary to make sure that the oil is completely removed. I recommend using a vinegar solution to douse the rock for several minutes. You can then rinse it thoroughly with water and let it dry naturally before use.
  • Combat algae – Algae tend to grow on all hard surfaces, including geodes. You can prevent algae deposits via several methods, like preventing overfeeding, cleaning and vacuuming the substrate, managing light levels, and even relying on algae eaters to keep the tank clean. Despite all these prevention methods, algae will eventually cover the rocks, even if in small quantities. These are notoriously difficult to remove due to the geodes’ more porous nature and overall structure. Algae will also grow inside the rock’s open cavity, making removing them even more difficult. You should clean the geode regularly to prevent algae overgrowth. You can achieve this by dousing the rock in a bleach solution, using carbon injections, and scrubbing the algae where reachable.
  • Verify effervescence – Effervescence refers to the rock releasing gasses, minerals, and heavy metals into the surrounding environment; in your case, in the water. You can test the rock to see if it presents any effervescent properties. The process usually involves the use of hydrochloric acid, but this isn’t widely available to the population. Vinegar is a good substitute, and pouring a few drops on the rock should do the job. If there’s fizzing and bubbling visible, the rock is effervescent, making it potentially unsafe for your tank.

Where to Find Geodes in Nature?

You can find geodes in the nature, but it’s mostly a game of chance. If you don’t live in areas where geodes occur naturally, you may have a lot of walking to do in the process.

Geodes are more widely spread in volcanic and desertic areas or regions high in limestone and other minerals.

Fortunately, you can find geode-rich areas, depending on the geode type and ease of access. California, Utah, and Arizona are some of the states that offer access to several geode sites, in case you’re interested.

The plus of getting the geodes straight from nature is that you have the freedom to choose whichever piece(s) you like.

The downside is that you may not possess the tools to process the rock.


Geodes are amazing pieces that will create breathtaking underwater effects due to the light refracting through the water column.

These rocks also come in a variety of shapes and colors with numerous crystal facets creating astounding light effects.

Unfortunately, many types of geodes are unfit for aquarium use. So, choose your favorite piece carefully and always be aware of the risks.

Author Image Fabian
I’m Fabian, aquarium fish breeder and founder of this website. I’ve been keeping fish, since I was a kid. On this blog, I share a lot of information about the aquarium hobby and various fish species that I like. Please leave a comment if you have any question.
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