How To Prepare Rocks For An Aquarium – All You Need To Know

    how to prepare rocks for an aquarium

    Are you looking to improve your fish tank set up with some budget-friendly aquascaping? Learn how to prepare rocks for an aquarium, and you can build realistic caves for your pet fish for pennies!

    To prepare rocks so that they are safe for a fish tank, you have to choose the right type of rock first. You then need to clean the rocks thoroughly and sterilize them. Placement strategies matter too!

    We’ll go over the most efficient rock cleaning and sterilizing methods in this guide.

    But, make sure you don’t skip over the sections related to the types of rocks you can, or shouldn’t, add into a fish tank.

    Introducing the wrong type of rock into an aquarium can cause a cascade of water chemistry imbalances.

    This can be just as harmful as introducing hitchhiking bacteria with an unsterilized rock.

    4 General Steps To Safely Prepare Rocks For An Aquarium

    how to make rocks aquarium safe

    You can source your rocks from the great outdoors while hiking or on a camping trip.

    If you’re into DIY-ing your way through the fishkeeping hobby, this is one of the easiest projects you can take on.

    Cleaning and sterilizing rocks for aquarium use is a pretty straightforward process.

    Rocks, especially porous ones, can carry bacteria, parasites, and other pollutants.

    For the safety of your fish, you’ll need to make sure the rocks are disinfected and residue-free before adding them to the aquarium.

    The trickiest part of preparing rocks for a fish tank is figuring out which type of rocks are suitable for your setup.

    The goal is to find neutral rocks so that they don’t trigger sudden changes in water parameters.

    A spike in water hardness, or a drastic fluctuation of the pH level, can cause a great deal of stress for your pet fish.

    Heightened stress levels in fish will leave them susceptible to diseases. 

    No décor element is worth risking your fish’s wellbeing!

    Luckily, there are plenty of fish-safe types of rocks for you to choose from.

    The 4 steps of this DIY project are:

    1. Choose the correct type of rocks for your fish tank.

    2. Clean the rocks thoroughly.

    3. Sterilize the rocks using a fish-safe disinfectant.

    4. Add the rocks to the fish tank without damaging your acrylic/glass aquarium.

    Where To Find Rocks For Your Aquarium

    Are you interested in building rock caves to give your fish new spaces to explore and use as refuge/resting areas?

    To collect aquarium-safe rocks, you first need to know where to look for them.

    Here’s a small list of locations where you can find rocks for a few bucks, or even free:

    • Dry creek or stream beds;
    • Beaches;
    • Lakesides;
    • Riversides;
    • Garden centers;
    • Landscaping businesses;
    • Gardening sections in hardware stores.

    Where Shouldn’t You Grab Rocks From?

    There are a few places where you shouldn’t grab rocks from:

    • Protected nature territories (nature reserves, national parks, etc.) – for ethical reasons, as these areas are home to fragile endangered species;
    • Polluted waterfronts – because fish-safe disinfectants won’t be efficient enough to make the rocks safe for aquarium use. 

    No matter where you collect the rocks from, cleaning and sterilizing them are unskippable tasks!

    To prepare rocks for an aquarium, you’ll need fish-safe cleaners and a bit of time to spare.

    Safe Rocks To Prepare For Freshwater Aquariums

    how to prepare rocks for a freshwater tank

    Rocks that you can safely add to a freshwater tank are easily accessible.

    They make great additions to your aquarium’s biological filtration system.

    Rocks can quickly get colonized by the culture of good bacteria that a fish tank needs to self-clean.

    Here’s a list of rocks that are considered safe for freshwater aquariums:

    • Granite;
    • Lava rock;
    • Slate rock;
    • Petrified wood;
    • Quartz;
    • Onyx;
    • Sandstone.

    Keep in mind that rocks are by nature made of countless grains of minerals.

    This means that even if a type of rock is considered aquarium-safe, it can hold fragments of minerals that might make them unsafe for a fish tank.

    Keen on a natural stone aquascape design, but you’re unsure about your rock identifying skills? 

    You can find some pretty impressive textured freshwater rocks online.

    Safe Rocks To Prepare For Saltwater Aquariums

    Finding rocks that are safe to add to a saltwater aquarium is trickier.

    The safest option if you want a water chemistry-friendly rock in a marine tank is live rock.  

    Live rock, or dry rock, is made of coral fragments that get broken off of a coral reef. These types of rocks are usually collected in shallow waters.

    They’re called “life rocks” because they make excellent habitats for beneficial bacteria, invertebrates, and lots of marine critters.

    Here are the most popular types of live rock that are safe for saltwater aquariums:

    • Fiji Rock;
    • Tonga Branch Rock;
    • Pukani Rock;
    • Aquacultured Live Rock. 

    Rocks You Shouldn’t Put In An Aquarium

    From their sourcing location to their mineral composition, there are a few types of rocks that you shouldn’t put in an aquarium.

    These rocks can either leach dangerous chemicals into your aquarium or negatively alter the water’s biochemical balance.

    Rocks from contaminated waters

    You can clean and sterilize rocks up to a certain degree, but fish-safe disinfectants won’t remove traces of dangerous pollutants.

    You shouldn’t add rocks taken from highly polluted water into your fish tank to be on the safe side.

    Rocks that were kept in other treated tanks 

    Rocks that were previously kept in an aquarium where medicine was used to treat fish shouldn’t be reused in other tanks.

    Porous rocks, in particular, can retain chemicals for a long time and can leach them out into the new aquarium.

    Rocks that crumble easily 

    Sedimentary rocks are the ones that will typically crumble when submerged underwater for long periods of time.

    So, even if the rocks look great when you’re collecting them, they should definitely stay out of your fish tank.

    Sharp-edged rocks

    Depending on the fish you’re keeping, rocks with rough or sharp edges can pose a threat to the inhabitants of your aquarium.

    Avoid adding these types of rocks to betta tanks or any other fish with long flowing fins.

    Rocks with visible specks of metal or rust 

    Metal deposits, or rust, ingrained into a rock are impossible to get rid of. 

    Don’t add rocks with visible shards of metal in them to your fish tank, as they are extremely dangerous.


    Calcite rocks will slowly decompose when submerged underwater.

    They might not crumble under your eyes, but they will release dangerous concentrations of calcium carbonate at a fast rate.

    Calcium carbonate released by calcite rocks will increase water hardness and make the pH level of your water fluctuate.

    Hardier fish in already harder water might not suffer any immediate consequences if you add calcite.

    But, for more sensitive fish in softer water, this can be fatal.

    Here’s a list of rocks you shouldn’t put in an aquarium:

    • Marble;
    • Geodes;
    • Alabaster;
    • Limestone;
    • Dolomite.

    How To Test Rocks For Fish Tank Safety

    How to test rocks for aquarium

    Testing rocks before adding them to your fish tank is highly recommended.

    Traces of limestone in sandstone are a crumbly disaster waiting to happen that can ruin your aquascaping plans.

    Test carbonate content

    An easy way to test rocks for calcium carbonate content is to splash them with a bit of vinegar.

    If the stone you’re testing is a calcite rock, the reaction will be visible immediately.

    The rock’s surface will fizz due to the chemical reaction between the vinegar and the calcium carbonate.

    In this case, don’t bother cleaning, and sterilizing, the rocks. They aren’t safe to use in an aquarium.

    Test porosity

    If you’re adding rock to your fish tank to give nitrifying bacteria more room to grow and spread, you should choose rocks with higher porosity.

    Visually, porous rocks will look rougher and will dry slower than smooth rocks when wet.

    Crumble test

    A crumble test is a good idea to make sure the rock you’ve collected won’t crumble while already submerged in your aquarium.

    Fill a container with old tank water, and soak the rocks for 24 hours.

    If they are still intact after this soak, they should be safe to keep in an aquarium.

    Test water parameters

    The best way to tell if a rock will alter the water chemistry inside your fish tank is to put it to the test in the same water conditions.

    • Submerge the rocks you’re testing in a container filled with old tank water.
    • Test the water’s pH level and hardness using pH test kits and carbonate hardness test kits. Write down the results because you’ll be using them to compare values later.
    • After a week, retest the water in your rock container. 
    • Compare the results. 
    • If there are significant changes in these two water parameters, it’s best that you don’t add the rocks to your fish tank. 
    • If the reaction is neutral, the rocks are safe to use!

    How To Clean Rocks For Aquarium Use

    Even the smoothest rocks need a thorough clean-up before the sterilizing phase.

    Porous rocks, and those with nooks & cracks, will require a bit of extra scrubbing.

    Here’s how to clean rocks to make them safe for a fish tank:

    • Submerge the rocks in hot water.
    • Scrub any dirt, or grime, off using a firm brush. 
    • For porous rocks or those with cracks, use a toothbrush to make sure you’re reaching every inch of the rock. 
    • Insects, or insect larvae/eggs, might be hiding in small crevices. Be as meticulous as you can if you’ve found a particularly holey rock. 
    • Move the scrubbed rocks in a container filled with clean water and repeat the scrubbing process. 

    Using soap isn’t recommended at this stage.

    Soap residue will be hard to get rid of, which can be highly harmful to aquarium fish.

    A stiff brush alone will remove the debris that would make your water go cloudy or murky.

    The next step in the rock prepping process is getting rid of any bacteria or living microorganisms settled on the rocks.

    How To Sterilize Rocks For Aquarium Use

    clean freshwater aquarium

    There are 3 popular methods of disinfecting rocks for aquarium use:

    1. Most efficient method – Bleach Soak;
    2. Safest method – Hydrogen Peroxide Soak;
    3. Controversial method – Boiling Rocks.

    Sterilize rocks with a bleach soak

    This method is the most efficient because it kills off all microscopic organisms that might be invisible to the naked eye.

    When done correctly, it’s just as safe as a hydrogen peroxide soak.

    Step-by-step guide:

    1. Go through the cleaning phase, scrubbing any dirt/debris off of your rocks. 
    2. Fill up a clean container using 9 parts water to 1 part bleach. Ensure the container is large enough to keep all the rocks fully submerged in the bleach solution.
    3. Soak the rocks in the bleach solution for 10-20 minutes (no longer than 20!).
    4. Rinse the rocks thoroughly using tap water.
    5. Fill another clean container with dechlorinated water (use a double dose of water conditioner).
    6. Soak the rocks in the container filled with dechlorinated water for 24 hours.
    7. Remove the rocks from the container and allow the rocks to dry for another 24 hours.

    This might seem like a painstakingly long process, but it’s the safest and most efficient way to sterilize rocks.

    It’s worth the effort when you get to use rocks as centerpiece aquascape props!

    Sterilize rocks with a hydrogen peroxide soak 

    A hydrogen peroxide soak is considered the safest way to sterilize rocks.

    That’s because, after 24 hours, hydrogen peroxide converts back into just oxygen and water.

    This method is highly recommended if you’re collecting rocks from water streams, and they’re covered in algae.

    Step-by-step guide:

    1. Go through the cleaning phase, thoroughly scrubbing any dirt/debris off of your rocks.
    2. Fill up a clean container using 5 parts water to 1 part hydrogen peroxide.
    3. Soak the rocks in the hydrogen peroxide solution for 30 minutes.
    4. Rinse the rocks thoroughly using tap water.
    5. Allow the rocks to dry for 24 hours.

    If your rocks have rough patches of algae on them, you’ll notice some fizzing while the rocks are soaking.

    Boil the rocks to sterilize them (use caution!)

    Boiling rocks to disinfect them is controversial because the consequences are simply not worth it if things go wrong!

    Rocks can explode and cause both damages and physical injuries.

    Porous rocks can crumble in the boiling phase, and you can wind up without any usable rocks by the end of it.

    You’ll also need to wear/use protective gear when handling hot rocks.

    Step-by-step guide: 

    1. Go through the cleaning phase, thoroughly scrubbing any dirt/debris off of your rocks.
    2. Submerge the rocks in a pot of cool, clean water.
    3. Bring the pot to a boil, and turn down the heat to a low simmer.
    4. Cover the pot, more so for your protection than to help the process along.
    5. Boil the rocks for 30 minutes.
    6. Remove the pot from the heat. At this point, you can allow the rocks to cool while submerged, or you can use kitchen tongs to remove them.
    7. Allow the rocks to cool completely, and you’re done.

    If you’d rather not take any risks, an alternative method is to pour boiling water over the rocks in a bucket or tub instead of submerging them in boiling water.

    You could pour the boiling water onto the rocks a few times, let them cool down, and then thoroughly scrub them to remove any contaminants.

    How To Safely Add Rocks To An Aquarium

    There are 3 basic guidelines to follow to prevent rocks from damaging your aquarium or injuring your fish:

    1. Secure your DIY rock fish cave using aquarium-safe glue.
    2. Avoid stacking rocks against the tank’s panels.
    3. Place rocks on a substrate layer rather than placing them on a bare tank bottom.

    Pros & Cons To Having Rocks In An Aquarium

    There are several pros and cons to having rocks in your aquarium.

    The Pros:

    • You can use rocks to prevent bottom dwellers from uprooting your live aquarium plants.
    • You can tie plant seedlings to rocks to keep them anchored while they develop a stronger root system.
    • DIY rock formations make excellent natural habitats for fish;
    • Porous rocks will become homes for more beneficial bacteria, improving your fish tank’s water quality.

    The Cons:

    • Rocks can change the water chemistry inside your fish tank and increase fish stress levels in the process.
    • Sharp-edged rocks are dangerous to have in a fish tank with clumsy swimmers. Bettas can easily tear their long fins simply when swimming by a rough-textured rock.
    • Improperly set up rocks can damage your aquarium and even injure your fish.
    • Rocks tumbling down can scratch, or even crack, a fish tank.


    Knowing how to prepare rocks for an aquarium before adding them is essential.

    Make sure the rocks you collect and plan on adding to your tank are fish-safe. 

    Test them using the methods listed above, prepare them for aquarium use, and then let your creativity handle the rest!

    Hardscaping using natural rocks is an inexpensive way to give your fish more spaces to explore and use as resting or refuge areas

    Rock aquascaping is an excellent choice for big community tanks where you need larger structures to divide the space.

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