Updated: Jun 12, 2022
Safe Rocks for Fish Tanks
Imagine dropping a sugar cube in a glass of water. It dissolves pretty quickly and now you have sugar water. Now imagine dropping a clump of salt in a glass of water. It takes a little longer, but eventually dissolves and now you have salt water. Finally, imagine dropping a piece of chalk into a glass of water. It doesn’t dissolve at first. It takes hours before you notice any perceivable decrease in size. Eventually though, the chalk dissolves completely and now you have calcium water.
This principle of erosion via dissolving solids in a solution is the same process rocks undergo in your fish tank. Some rocks are formed in such a way that it takes centuries to dissolve. These are referred to as “hard” rocks. Others start dissolving in hours or days and are called “soft” rocks.
Just because a rock will dissolve in your tank doesn’t mean it's unsafe. Some rocks are what we call “inert”. This means their chemical composition does not affect the water chemistry of your aquarium. Other rocks are reactive, meaning they are composed of chemicals that will alter your water chemistry.
For the purposes of this article, we can define a “safe” rock for the aquarium as inert and preferably hard. An unsafe rock, therefore, can be defined as reactive and soft. In this article we will discuss which of these categories some of the more popular aquarium rocks fall into, and how to determine if lesser known or found rocks are safe or not for your fish tank.
When I say “hard rocks”, I am referring to the type of rocks that are more dense and not easily dissolved. These types of rocks are usually igneous or metamorphic, but sometimes sedimentary. Igneous rocks are formed when molten lava crystalizes and then solidifies either very slowly deep in the earth, or very quickly near the surface. Some metamorphic rocks can be defined as harder because they were once igneous or sedimentary and then underwent change from immense heat and pressure.
Some examples of harder rocks are; quartz, granite, slate, and obsidian.
When I say “soft rocks”, I am referring to the type of rocks that are less dense and dissolve in water relatively easily. These types of rocks are usually sedimentary or metamorphic, but can be igneous. Sedimentary rocks are formed when existing rocks loosen and accumulate in layers that are eventually cemented over time. Some softer metamorphic rocks are formed after sedimentary rocks undergo a physical or chemical change, usually heat and pressure. These types of rock are softer because they are formed from rocks that are already susceptible to being loosened or eroded.
Some examples of softer rocks are; sandstone, limestone, shale, mudrock, and pumice.
Inert vs Reactive Rocks
I define inert rocks as those that either do not dissolve into the water or are made up of a chemical composition that does not react with the water they are placed in.
Reactive rocks are defined as rocks that do dissolve in a fish tank, but more importantly, significantly alter the aquarium’s water chemistry. The most common example is limestone. Limestone is made of calcium carbonate, (CaCO3). As this rock dissolves in the water, it raises the carbonate alkalinity, or carbonate hardness (KH) of softer water, which raises the pH. Carbonate hardness is the concentration of carbonate (CO3) in a solution, in this case aquarium water. The calcium (Ca) portion of calcium carbonate will raise your general hardness (GH) as well. If you have a saltwater or hard water tank, the GH and KH won’t raise as quickly or significantly as the concentration of CaCO3 in the water is already high.
Rocks can be hard and reactive, hard and inert, soft and reactive, or soft and inert.
Popular Aquarium Rocks
Most of the rocks used in saltwater aquariums are either real live rock, or synthetic live rock. Synthetic live rock is inert and doesn’t dissolve in the aquarium. Real live rock is hard and reactive. It is made of aragonite, a crystalized form of calcium carbonate, derived from dead coral skeletons. Live rock, being crystalized, does not dissolve easily, but is reactive and will release calcium carbonate into the water, raising the pH, GH (general hardness), and KH (carbonate hardness) if the water is softer. Sea water is considered harder water and has a higher GH and KH.
Rocks used in freshwater tanks on the other hand come in all shapes, sizes, and compositions. We’ll explore a few of them here.
I classify slate as a hard, inert rock. It is a metamorphic rock formed from shale and low grade pressure. Slate is composed of 95% mineral quartz and crystalized clay. It will not completely dissolve in water because the clay has been compacted and the mineralized crystals remain microscopic. Slate does not release any reactive chemicals into the water column, although some slate may contain iron or carbonate minerals, but in very low quantities. Slate with iron present can be identified by patches of green. Most slate is safe for any aquarium and works great to create shelf-scapes and overhangs due to its wide, flat shape.
Most types of available river rock are assorted igneous, metamorphic, and sedimentary rocks that have been eroded and shaped by active streams. Due to this process, most river rocks are harder as softer rocks would erode completely. For this same reason, I would consider most river rocks to be inert as well. River rocks come assorted, or as single types of rock sorted by location.
Dragon stone, or ohko stone is a sedimentary rock made of mostly clay and organic matter. It’s unique pattern is formed from waves depositing clay minerals and tides forming the distinctive pockets. Ohko stone is light weight. I would consider it an inert, soft rock. It can dissolve easily and must be thoroughly rinsed before adding to the aquarium as clay dust will accumulate on the outside. It would still be considered inert as the clay that dissolves does not change the water chemistry.
Seiryu stone is a metamorphic, limestone based rock. It has calcium carbonate in its composition, but is very dense so it dissolves slowly. Seiryu stone can affect the GH, KH, and pH of your aquarium water, especially in smaller aquariums. It won’t affect water chemistry as drastically as limestone due to its metamorphic nature, the rock is much denser and does not dissolve as readily as limestone.
Texas Holy Rock
Honeycomb Limestone, or Texas holey rock (THR) is a type of limestone from Texas that gets its holes when acid from cypress tree roots seeps into the bedrock below. As limestone, THR, is mostly calcium carbonate and will significantly raise a fish tank’s general and carbonate hardness, and pH. With its bright white color and many holes of varying sizes, it is ideal as a backdrop and habitat for the brightly colored and territorial African cichlids. THR is a soft, reactive rock.
Granite is a hard, inert rock. It is igneous and forms from cooling lava flows in mountainous regions. Granite is made of mostly silica crystalline quartz with some potassium and calcium. It will not dissolve in neutral water, but can become weak and chip away in acidic, or high calcium concentrated water. Granite comes in all kinds of patterns and colors. It's a dense and heavy rock and shouldn’t be placed directly on glass.
Quartz is a popular aquarium rock due to its extreme inertness, hardness, and unique composition. The crystalline structure is appealing and visually distinct from other popular aquarium decor. Quartz does not exactly fall in line with a natural aesthetic, but is perfectly safe for the aquarium as it is primarily composed of silicon dioxide, an inert compound in water.
Sometimes you are hiking in the woods, near a stream, or just visiting a local garden supply store and you discover an amazing looking rock. Inevitably you ask, will this rock be safe in my aquarium? To answer this question you must consider several factors.
What type of aquarium is the rock going into; saltwater, hard water, or soft water? A reactive rock made of calcium carbonate shouldn't go into a soft water tank, but would be fine in a hard water aquarium.
You can test the approximate amount of calcium carbonate in a rock by applying white vinegar to the piece. Usually, some fizzing will occur as the acidic vinegar reacts with the alkaline rock. This test is not full proof, but is an easy way to quickly eliminate some options.
Does the rock chip, break, or dust easily? If the rock in question chips or breaks easily, it may still be inert, but if it powders easily, this could be indicative of a soft rock. Soft rocks are not a deal breaker, but they can be if they are also reactive.
A popular found rock in most regions is known as fieldstone. Fieldstone is popular because of its natural look. Fieldstone can be made of granite, sandstone, or limestone. The first two are safe, but as we’ve discussed, limestone is made of calcium carbonate and should only be used in a harder water tank.
If you find rock that has not been conveniently labeled by a garden supply store and is not easily researchable, the best way to determine if it is safe for your aquarium is to test it yourself. This can be accomplished by taking a small amount of your aquarium water in a separate container, and place a piece of the rock in question into it. Be sure to test your GH, KH, and pH before adding the rock. Then, test these parameters again after one, five and 10 days. By this time, you should see some change in water chemistry if there will be any.
There are many types of rocks out there and if you find one that you love, but it is not safe for your aquarium, keep looking. Many rocks will look like others without being reactive or too soft.
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