Can I use geode in my aquarium?

There is no doubt that landscaping is as much a science as it is an art. Imagination is your only limit when creating a unique, vibrant and breathtaking aquatic environment.

Today, we are going to bring you a little more about the geode. Is this rock safe to put in an aquarium? What exactly is a geode? As you can see, we have a lot to discuss here! So let’s go!

What is a geode?

Simply put, the geodes are rocks with crystallized minerals such as quartz, precious silica, opal or rhodochrosite, to name a few.

They are some of the prettiest decorations you can put in your aquarium, and they offer immense diversity in terms of shape, size, and coloring.

Geodes are very popular in homes as most people collect them for decorative purposes.

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

1. Volcanic geode

As the name suggests, the volcanic geode is the result of volcanic activity. Lava contains a variety of minerals such as quartz and opal, which are the result of mixing lava with groundwater. The mixture creates massive vapors that remain trapped underground as the lava cannot escape to the surface.

The steam creates voids in the molten rock, which can cool over time. The result is a crystallized geode, which is a heart of mineralized crystal covered by a hard layer of molten lava.

2. Sedimentary geode

Sedimentary geodes are the result of time, natural erosion and pressure. These rarer rocks are formed by elements such as carbonate, opal or quartz materials. They are usually created around empty structures such as tree trunks, shells or any other hard materials). These geodes are also very different in shape, content and size, except that they are smaller than volcanic formations.

Geodes are difficult rocks to cut because they are so hard. Its outer bed is notoriously difficult to cut and is a job meant for specialist machines. This formidable durability is also why geodes often remain sharp and unpolished, but more on that below.

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Are geodes safe for fish?

Yes and no! It depends on the type of geode we are talking about. Quartz and amethyst are generally safe because they do not change the water chemistry.

However, most sedimentary geodes come from limestone formations, which means that they have a high content of calcium. This calcium will seep into the water gradually, raising the pH over time. The rise in pH can have potentially devastating consequences if you have fish that prefer acidic water.

So be very careful with the geode you are adding to avoid dangerous fluctuations in pH it is us parameters in general. If you don’t want a pH-altering geode, you should skip limestone-based pieces altogether.

There is another issue worth discussing. Some geodes are impure and may be contaminated with heavy metals such as copper, zinc or even arsenic. These products are toxic to fish and any other aquatic life.

Therefore, to add a geode to your aquarium you need to make sure it is safe. So, let’s discuss about it!

Given what we’ve discussed about geodes so far, it’s understandable why you can’t just buy one and throw it into your aquarium. You must carry out some procedures to ensure that it is safe for fish and other inhabitants.

To be sure of this, consider following the steps below:

1. Find the right type of geode

The type of geode you are buying makes a big difference in this regard. As we discussed before, limestone-based geodes will affect the pH of the water. Therefore, you can use them in specific setups with fish that don’t mind the higher pH levels, for example, african cichlids and viviparous species, guppies, platys, swordfish and etc. This does not mean that limestones are completely safe for them. These rocks can raise pH levels beyond what these fish can handle. Keep an eye!

2. Watch your sharp edges

Many geodes are hard and rough on the surface. It is not uncommon for them to come with sharp, pointed edges, which can cause serious injury to the fish. Larger species or with longer fins, such as kinguios and bettasfor example, can have their fins and bodies pierced due to the unpolished appearance of the rocks.

You should always take a good look at the rock to make sure it is safe for fish. Geodes are extremely hard, so you won’t be able to polish them as easily.

3. Do a thorough cleaning

All geodes are processed prior to sale. The process involves cutting the rock to show its interior cavity and exposing the crystals and minerals. The problem is that the cutting process involves the use of a type of oil that can contaminate the aquarium water.

Thorough cleaning and sterilization is necessary to ensure the oil is completely removed. I recommend using a vinegar solution to soak the rock for several minutes. Then, you can rinse it off with water and let it dry naturally before using.

4. Check the effervescence

Effervescence refers to the release of gases, minerals and heavy metals from the rock into the surrounding environment; in our case, water. You can test the rock to see if it has any effervescent properties. The process usually involves the use of hydrochloric acid, but it is not readily available to the public.

An alternative to acid is vinegar, just pouring a few drops of it on the rock. If there is effervescence and visible bubbles, the rock is not safe for the aquarium as it can change the water parameters with time.


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

These rocks also come in a huge variety of shapes and colors, with countless crystal facets, creating surprising light effects.

Unfortunately, many types of geodes are unsuitable for use in aquariums. So choose your piece carefully and always be aware of the risks.