How to Choose the Right Aquarium Rocks and Substrate for Saltwater and Freshwater Tanks

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How to Choose the Right Aquarium Rocks and Substrate for Saltwater and Freshwater Tanks

You found a cool rock and thinking about adding it to your fish tank?  Aquarium rocks can come from various locations; you must make sure it is safe. 

We are not all geologists, and the best way to test fish tank rocks is to fill a container with water.  Allow it to sit overnight to remove the chlorine (if you are not using siphoned fish tank RODI or otherwise prepared water).  Establish base test results before adding the cleaned rock and wait for two weeks.  Test the water for any increase of elements. 

Freshwater aquascaping enrich the habitat for your fish and can be anything from plants to vast Chiclid rockscapes.  Saltwater aquarium rocks provide biological filtration for those not wanting to add a sponge filter to the fish tank.  Not all stones are suitable to become aquarium rocks due to the mineral deposited during formation. 

The chemical and mineral composition changes the parameters of water; saltwater aquarium rocks and freshwater fish tank rocks are different. Knowing the fundamentals of rock types, where to find them, and how to prepare them for both freshwater and saltwater fish tanks is vital in keeping the water parameters stable.

Once you have found the correct rocks, decorating and constructing secure rock structures are at the heart of it the same.  If you want full control over the construction, you can make your own.

Types of Rocks

The formation of rocks influences the chemical composition, and mineral elements found in the stone are divided into three main categories of rocks. Rocks may have mineral or ore deposits, which will make them toxic for most aquariums.

Igneous Rock

Igneous rocks are formed by solidifying magma deposited, above or below ground, and underwater.  The earth’s crusts melt when temperature increases, pressure decreases, or composition changes.  Once melted, the lava cools and solidifies to form rocks or natural glass.  Geologists have many uses for these rocks, but fish keepers only need to understand the chemical composition to determine if it is safe to use as aquarium rocks.

Lava rock aquarium can be granite, gabbro, or diorite.   Different textures develop in the way the lava erupts, cools, and composition of the rock.

Sedimentary Rock

Sedimentary rocks formed over time as particles (Biological or mineral) settle in an area to create layers of strata combined under pressure and compact and become solid.  Four classifications include clastic, biochemical, chemical, and others.  Many of these rocks have elements that can be harmful to a fish tank.   These elements can be ion phosphate and other minerals that can change the water chemistry.

Younger rocks have not solidified and may break apart; this is not suitable as aquarium rocks.  The texture fashioned by the solidification process and determines the density, porosity, and permeability of the stone.  Most fossils, comprised mainly of bones, shells, and woody tissue, are found in the sedimentary rock, and some of this organic matter may only be visible under a microscope.  Sedimentary fish tank rocks increase the pH of your water.

Also read – The Advantages of Adding an Aquarium Airstone

Metamorphic Rocks

Metamorphic rocks are reformed rocks.  The reformations occur because of contact metamorphism, where the lava’s heat has melted the stone.  Or dynamic metamorphism rocks subjected to pressure and temperature from the earth as layers are built and change the chemical composition of the rock.  Metamorphic rocks include slate rocks, marble, schist, and quartzite.

Where to Find Aquarium Rocks

Fish tank rocks can be bought online; from local pet stores, these shops cater to different hobbyists check that it is labeled aquarium safe. Unusual stones can be found in nature.  In both instances, prepare and test before adding to ensure that they are reliable rocks for freshwater aquariums or saltwater fish tanks.

Aquarium rocks should not leach unwanted chemicals into the water of your fish tank and change the chemistry, including the hardness of the water. Smooth round rocks form naturally through erosion. Stones for any aquarium should be smooth and rounded to prevent injury to fish and snails.

Aquarium rock décor is treated differently depending on the type of rock.  Lava rock in aquariums is treated and cleaned differently to ordinary aquarium stones and live rocks used in saltwater fish tanks. Fish tank rocks need to be prepared and tested before adding them to your fish tank.

How to Prepare Rocks for a Freshwater Aquarium

Preparing non-permeable rocks:

  • Scrub the aquarium rocks with a wire brush and detergent
  • Examine the stones for veins and shiny deposits that may be trace metals and will decimate your fish tank
  • Use sandpaper to smooth away any rough edges
  • Place in a pot with boiling water changing the water at 10-minute intervals
  • Boil until the water is clear
  • Place in siphoned off aquarium water until the rock is cool

Discard any potential fish tank rocks that crumble or reveal a rust color during the cleaning process.  They are not suitable as aquarium rocks and may add harmful elements to your fish tanks.  All aquarium rocks should be non-toxic, test to see if it is aquarium safe. 

Calcareous rocks contain calcium; this will affect your pH level.  Pour acid onto the rock if it bubbles and forms a foam it contains calcium. 

Aquarium Rocks – Adding Them To Your Aquarium

How to Prepare Rocks for a Saltwater Aquarium

Prepare rocks for a saltwater fish tank depending on what type you select live, dry, or mined.

Live rock has microorganisms in the rock and algae and bacteria with polymer slime; the biofilm settles in layers and the correct microflora and organisms to stimulate the growth of corals.  Many fish tanks need live rock but may host pests like fireworms, bubble algae, and aiptasia.

Also read – The Best Algae Eaters for Any Freshwater Aquarium

Live rocks can be harvested from the sea, manufactured, or mined where calcium rock or rubble that colonize bacteria.  Examine the rock for signs of decay of the micro biofilm when it arrives as it takes 30 days to transport the stone, and this is harmful.

Dry rocks have most biological elements removed before shipment but inspect the stone to ensure that it is free of all material and pests.  The biofilm has to be activated before it becomes a biofilter.

The mined rock is the safest option of the three as it comes from land and not the sea and is less likely to cause problems later.  The mined rock is composed primarily of calcium carbonate and contains almost no biological matter.  The mined rock is denser and takes longer to fill with water; however, cures quickly.  Mined rock does not come for the ocean, making it cheaper and environmentally friendly. 

Clean all rocks before you add them to your fish tank.

Curing Live Rock

The curing process can take weeks or even months before it is completed.  This process cannot be rushed.

How you clean dirty live rocks:

  • Step #1: Prepare a bucket of saltwater and heat to 78⁰ F
  • Step #2: Add an air pump to circulate the water
  • Step #3: Use a soft toothbrush to remove all decaying matter
  • Step #4: Use a tweezer to remove all worms and insects from the rock
  • Step #5: Place the stone into the bucket

For wet fish tank rocks, test the water weekly for ammonia, and complete water changes until there is no smell ammonia levels are zero.

For dry aquarium rocks, complete a full water change at four weeks (halfway) and test the water levels two weeks later when the nitrites and nitrates levels are zero. The rock is cured.

For mined rock, you can skip step #4 and leave it in the bucket for four weeks to develop a biofilm and add it to your fish tank. 

Side note: Don’t rinse live rock with tap water chlorine will kill the bacteria and destroy the rock.

Aquarium Rocks for Saltwater Fish Tanks

We will not recommend live aquarium rocks here as these rocks need to be purchased locally and delivered within 24 hours.

Caribsea Life Rock



The Caribsea life rock has 50% open space and is one of the durable natural aquarium rocks with good porosity, safe for both marine, reef fish tanks.  The natural weathering looks fantastic.   The shapes range; some rocks have bonded together to form rounds, arches, and cave structures.  These rocks have dormant bacteria that, once soaked, will activate and grow.  As it is natural, rock corals will take to it.



Our Verdict

Aquarium rock will take time to establish a biofilm, glue broken pieces back together.  This problem occurs with most dry rocks.  The holes are perfect for small fish, and it looks stunning overall.

Nature’s Ocean Coral Base Rock 4-8 INCHES



An aragonite rock perfect for growing live coral.  Pre-soaked to prevent leaching, ideal for saltwater aquariums. Even though the manufacturer states you add it to your fish tank straight away, cleaning rocks before adding them to the aquarium is always recommended.



Our Verdict

Aquarium rock, the smaller pieces, give you more options to build the creation you want.  The color appears bland in the beginning but will develop over time.  If you are growing corals to cover the rock, it is perfect. 

Emours Aquarium Imitation Coral Reef Rock Base



This manmade rock is made from durable, non-toxic resin.  The dark color will look perfect in a fish tank from day one.  Artificial aquarium rocks have holes, rounds, and caves for your fish to interact with and take care of the preparation for you.  Not suitable for corals, it will work as biological filtration; mined, or natural stones are better with corals and reef tanks.



Our Verdict

For marine hobbyists who do not have a coral fish tank, non-toxic fish tank rocks are a good option.  A bit small in larger fish tanks, but if you are using it as part of little islands in a long tank, it will look pretty. 

Red Sea Fish Pharm

On all dry rocks establishing the biofilm is essential, and for saltwater fish tanks Reef mature pro kit is ideal for speeding up the process.

Reef mature pro kit has all the biological elements needed to speed up the bacteria growth and maturity for your fish tank.  This kit includes Bacto Start, Nitro Back, NO3: PO4-X, and KH Coralline Grow, that a reef tank will need to establish the biological filtration of your fish tank.  To be used as part of your dry aquarium rock for saltwater fish tanks.

Saltwater Aquarium Sand

Sand is the base of many saltwater fish tanks; this is added after rock structures to keep it stable.  Calcium carbonate is essential in coral aquariums, but many marine fish tanks will benefit from the addition.

Nature’s Ocean No.0



Nature’s Ocean No.0 is live sand containing a high concentration of useful bacteria.  Suitable for deep saltwater aquariums with a moderate flow aimed above the substrate.  The fine granules made from calcium carbonate have minerals and essential elements that keep the fish tanks’ pH levels stable.



Our Verdict

This sand has all the properties you need in your fish tank, as a base it is great but needs a denser top layer to keep it stable in an aquarium where the flow is high or aimed at the substrate.  Perfect for delicate fish tanks.

Carib Sea Arag-Alive


The Carib Sea Arage-Alive Fiji Pink Sand granules are larger than that of Nature’s Ocean No.0, making it suitable for fish tanks with stronger water flow and comes with all the benefits of bacteria and trace minerals. 



Our Verdict

Head to head against Nature’s Ocean No.0  Carib Sea Arag-Alive is more suitable for most fish tanks.  The light color is perfect, and the living stand may take longer to settle and remove the cloudy appearance after a day it is complete.

Freshwater Aquarium Rocks

Natural Slate Stone 3 to 5 inch Rocks


Natural slate is fantastic in all types of fish tanks and terrariums.  The colors may vary in the package, but it does not leach anything into the tank and is easy to clean.  The flat surface is easy to stack and glue together, and you can build elaborate stable structures—sand down the sharp edges of slate to prevent hurting your pets.



Our Verdict

Although grey can be a bit dull paired with lighter stones to make them pop.  The safety and versatility of the slate stones are fabulous.  You can build one long structure or a few smaller ones depending on the size of your fish tank and what you want to add.

Lifegard Aquatics 25G-Smoky Smoky Mountain



Lifeguard Aquatics are pretty cool looking aquarium rocks; dark and light colors will accent plant effectively and with nooks and hiding holes that increase the surface area.  You arrange the pieces in your fish tank, and your fish can swim under around and hide with the hardscape.



Our Verdict

The texture and colors create can be used to create fascinating mountain structures. You do have to play around with it to get the pieces to fit together.  The product description is a bit disappointing to the contents, but it gives you more options.

Ceramic Aquarium Rock Cave Decor

Ceramic rocks are manmade and come in many different designs.  When adding caves and for fish, manmade is often easier.  These rocks are complete with escape hatches your fish can swim through if they need to but have a cave structure to help them feel safe.

Freshwater Aquarium Sand

#Nature’s Oceans African Cichlid



This sand has pebble-like aquarium stones that have a coarse appearance that provided additional biofiltration and will not add any harmful elements to your fish tank.



Our Verdict

The chunky dark appearance of this river sand effortlessly set off lighter rocks and sand a perfect combination with bright fish.  Varied textures and colors attractive in freshwater ponds and fish tanks, although the result is a dark aquarium.

Pure Water Pebbles African Cichlid Bio-Activ Natural White Aquarium Live Sand



White pebbles filled with the needed bacteria.  These stones are heavy and large if used on its own a little bland with color and texture.  It provides excellent biological filtration, and the large sizes are easier to clean.



Our Verdict

This white pebble is large and works perfectly on its own or paired with a sprinkling of Nature’s Oceans African Cichlid for added lightness to brighten up the fish tank with light colors.  Head to head; it is hard to decide which one is better, blending them with the white base, and additional darker elements are ideal

Fluval Plant and Shrimp Stratum



The Fluval Stratum is fertile volcanic soil that has all the nutrients for your plants.  This substate is suitable for freshwater planted fish tanks that need to be more acidic.  It provides a good purchase for the aquatic plants and enough nutrients to stimulate plant growth.



Our Verdict

For more acidic fish tanks, this substrate is perfect, compare the pH requirements of your inhabitants before selecting this as a substrate.  Using another type with fertilizer is another choice of aquariums with different needs.

Homemade Fish Tank Decorations

Even bought fish tank rocks may need to be altered to enhance its beauty.  With a few home tools, glue you can turn any flat rock into something wonderful.

Driftwood and Stone Aquascape Ideas

Hardscape aquariums are easy to create; a few chisels and a hammer can change any rock into a formation you want.  Cut and straighten the bottom for more stable base stones.  Include hiding holes and caves for your fish.  Hard stones can be glued together for aquarium caves.

Key elements when planning your structure:

Aquarium rocks and other hardscape impact the flow of water inside the fish tank.  The direction, speed, and placement of overhead pumps may have to be adjusted to eliminate dead spots behind and in between the rocks.  Corals, plants, and fish need different current strengths and need to be considered before you make these adjustments.

Think about the cleaning of the aquarium before you decide where to place fish tank rock, which inhibits the cleaning of glass and other elements.  Place rocks near the middle and not against the glass, allowing space for cleaning tools.

Aquascaping requires long term planning the height of rocks in comparison to the height of your fish tank today will look different compared to what it will look like after corals or aquarium plants grow, think about the space in the future.  It is best to build rock walls that are about halfway up or two thirds from the water level.

Aquarium rock walls are not the only options.  Space rocks around the tank with other elements like encrusting corals.  Smaller rock pieces can be used as islands creating separate fish territories, swim-throughs, and open spaces.

The design principle of 3 works creates a balanced background.  Divide the tank into three vertically and horizontally and add focal points on these lines.

Keep your fish tank glass and plants and corals safe by gluing the rocks together.  Superglue works well, and once dry will not add any harmful elements to your fish, leave it to dry for 24 hours.  Work with dry rocks and match the color to what the future rock color.  Build up foundation stones with flat bottoms like slate rocks and fill gaps and cover glue with smaller stones.  Secure large fish tank rocks with fiberglass or acrylic rods for support.

What Aquarium Plants Grow on Rocks

A list of plants that can be glued to rocks in planted aquariums

  • Microsorum Pteropus ‘Narrow.’
  • Anubias
  • Java Fern
  • (Java) Moss
  • Hemianthus (Dwarf Baby Tears)
  • Bolbitis Heudelotii (African Water Fern)

How to Attach Aquarium Plants to Rocks?

There are two ways to attach plants to a decoration or rock in your aquarium.  You can push it into a hole or indentation and secure with a string until it takes hold or glue it with a small amount of oasis florist glue.

How to Grow Moss on Rocks in the Aquarium

Place glue in different areas of the cleaned and prepared aquarium rock.  Pull pieces of moss and hold it on the glue for a minute but allow more time to dry.  Keep the moss moist as the glue dries.

How to Make Live Rocks

DIY rocks for a freshwater aquarium is simple.  Cement will leach elements into the water, and it takes two months to cure replace the weekly water test every couple of weeks until elements return to normal.  DIY aquarium rocks are perfect for building fish caves for an aquarium, and although plants can be attached to cement rocks, these are not suitable for corals.

Required products for DIY aquarium rocks

  • cement grey or white
  • crushed coral or shells for texture (optional if you can have harder water)
  • rock salt
  • aquarium or reef sand as a mix for the rock
  • playground sand to create a mold
  • Two mixing containers, one large

In the smaller container mix 1 part sand, 1 part crushed texture two parts cement add water slowly in small quantities until the mixture is the consistency of peanut butter.  Add two parts of rock salt to make the rock porous.

In the end, fill the larger container with the playground sand and add water until it holds its shape.  Remove enough to cover the top of the cement once the rock formation is finished.  In the sand form, the inverse of the design you want to create in your aquarium rock, including peaks and valleys and some branches.  Add some texture to the sand with stones and household items.

Fill in the valleys and hills with cement flatten the base.  Cover with wet leftover playground sand.  Let the cement dry add about two days to manufactures specifications but keep the sand moist with a spray bottle.

Once sufficient time has passed for the cement structure to cure, excavate the rock from the sand and rinse the sand off from the aquarium rock.  Allow the rock to dry for 1 – 2 days then color the rock with PVC primer.  Add layers of primer until you achieve the desired cover allow the stone to dry overnight before you prepare it for the fish tank.

Keep the rock in fresh or saltwater until the elements return to normal before adding the aquarium rock décor to your fish tank.

FAQ’s on Saltwater and Freshwater Aquarium Rocks

How Much Rock Do You Need For Your Fish Tank? The amount of rock most aquariums need is about 1lb of stone per gallon of water.  If you want less rock visible in your fish tank, you need to introduce biofiltration in the hidden spaces of your aquarium unit.  The rock’s shapes and sizes are different, buy 20% more stone than you need to create an attractive aquarium hardscape.

What’s the Best Dry Rock for My Nano Tank? In Nano tanks, be careful about the mineral content, which will impact the habitat.  Rocks without calcium, salts, and trace elements are advisable.

When selecting aquarium rocks for small, unforgiving fish tanks, select dry mined rock for saltwater fish tanks or slate rock for freshwater ones, which cure quickly and do not contain any biological materials.  It will take about eight weeks to build a biofilm, but as the rock matures, the biofiltration will be available, and your fish tank rocks will not add any harmful or unwanted chemicals. 

Select live rock that has a fresh ocean smell and cure well; if this is the root you want to go, any leaching element that dilutes into a small amount of water is costly.  Parasites can overwhelm the nutrient level and destabilize the tank.

The amount of live rock you need is about a 1lb of rock per gallon of water; another biological source of filtration is recommended with less rock to prevent the restriction of water flow. 

Can Heavy River Rock Break a Fish Tank? Aquarium rocks that move against the glass can scratch it, and if it falls, it may break it.  Keeping fish tank rocks away from the glass, and a good layer of the substrate will keep the rock from scratching the glass.  Keep the structure stable with flat base foundation stones and extra glue.  It takes 24 hours for most adhesives to set this will help prevent the rocks from falling.

What Rocks Raise pH In Aquarium? There are a few culprits, calcareous rocks that contain calcium, magnesium, dried coral, or rocks that have shells in it.  These rocks are usually sedimentary rock formations and are not suitable as aquarium rocks.  Unstable pH levels in a fish tank are dangerous.  However, reef tanks need extra calcium, and in this instance, calcium carbonate rocks are preferred.  Testing rocks before you add them to your fish tank is vital.  If these tests match your fish tank, it is safe to use.

A shortlist of calcareous rocks

  • Limestone
  • Dolostone
  • Chalk
  • Porphyry

If you are making rocks check the ingredients limestone is a component of many products.

The Purpose of Aquarium Rock Décor

Aquarium rock décor is not just visually appealing, but rocks naturalize the scenery, supply hiding places, and shade for your fish.  Some fish devour the debris collected between the crevices and is a natural part of their diet, and all the while, these seemly neutral pieces are an essential part of keeping your fish tank safe within the nitrogen cycle. 

Selecting the correct rock that does not harm the environment is critical.  Think about the long-term growth of plants or corals when choosing and combining fish tank rocks, aquarium stones, and substrate.  All of this depends on the fish and what parameters they require. 

When selecting aquascape rocks, I prefer natural mined rocks in saltwater aquariums but ceramic and or slate rocks for freshwater fish tanks.  Easy to build up and glue together in any shape you want with a bit of preparation, you will have a stunning aquarium hardscape and ensure that water pH level remains unaffected.

Also read – How to Choose the Best 55 Gallon Fish Tank on the Market

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