Rocks are an essential decor item but several kinds will affect your aquarium water chemistry. The minerals that some rocks contain will dissolve in water. These chemicals will raise the pH, depending on the kind of rock.
What Rocks Raise pH in Aquarium?
Sedimentary rocks like limestone, sandstone, and conglomerate will raise the pH. They contain calcium carbonate (CaCO3), which is soluble in water when low levels of acid are present. Plant tannins, carbon dioxide, and other weak acids found in freshwater tanks will dissolve minerals out of the rocks you add.
Some metamorphic rocks, like marble, will raise the pH as they are made of recrystallized calcium carbonate. Others, like gneiss, slate, and schist are safe for aquarium use.
Igneous rocks like granite, basalt, and lava rock are the safest as they contain little to no carbonate minerals. They are composed of silicate minerals instead. Blue sodalite stone is also safe since it is silicate-based. As are any kind of quartz-based stone, including quartz crystals, polished “river stones,” citrine, rose rock, and amethyst.
Beware of rock painting, however. Some rock dealers dye or paint over plain quartz crystals to make them look like amethyst and other, more desirable, rocks. Rock dyes may be toxic to your fish. And even if they are non-toxic, the dyes won’t last for long when submerged in your fish tank.
Does Limestone in Aquarium Raise pH?
All limestone rocks will raise the pH of your aquarium water. But that does not mean the rock is unsuitable for aquarium use.
Many species of fish prefer a high pH. African cichlids from Lake Malawi and Tanganyika require a pH of 7.8-8.5. Livebearing fish like platies and mollies come from mineral-rich water and also prefer a higher pH.
You want to use rocks like limestone to help buffer the water. Buffering, also known as alkalinity, is the ability of water to resist changes in pH. Using whole or crushed limestone is an excellent way to raise alkalinity in aquariums. Crushed limestone is a common substrate for reef tanks as well.
Does Dragon Stone Affect pH?
Dragon stone is one of the most in-demand rocks for aquarium use. It is important for Nature and Iwagumi-style fish tank aquascaping, popularized by Takashi Amano in the 1990s. The lacey structure is unique and offers excellent attachment points for aquarium moss and plants that attach to hard surfaces.
Dragon stone will raise your pH but not as much as limestone will. That property makes it useful for mineral-poor setups where you want the pH to remain either neutral or slightly acidic. Driftwood or other sources of tannic acids will balance out the mineral content to keep your water soft.
Does Seiryu Stone Raise pH?
Seiryu stone is just as popular as dragon stone thanks to its dark and light striped pattern. Nature and Iwagumi aquarists buy it in huge amounts but it’s becoming a staple for aquarists of all skill levels now.
Seiryu stone is a metamorphic variety of limestone (marble) and has high amounts of calcium carbonate. Therefore, seiryu stones will raise the pH of your aquarium water.
One way aquarists deal with this is by treating it with a strong acid. A 30% solution of HCL (hydrochloric acid) or muriatic acid will react with all of the exposed CaCO3. You can then rinse and allow your treated stone to soak for a few days to remove any acid traces.
It is safe to place into your aquarium now and it will release little to no pH-raising chemicals. Another benefit of acid-treating seiryu stone is that the colors become even deeper. Rather than gray and cream the acid etches the surface to a shiny black and white look.
How to Test Rocks for Aquarium Use
If you aren’t sure about the kind of minerals a given rock has, you should test it before adding it to your tank. For those with plenty of time, the easiest way is to first clean the rocks to remove any dust and dirt.
Next, you need to fill a bucket with enough aquarium water to cover the entire rock. You shouldn’t use treated tap water, which is often high in minerals from the start. These may prevent rock minerals from dissolving, unlike your aquarium environment, which contains more acids.
Place your rock inside the bucket and let it sit for a week indoors. Test the water’s pH one day after adding your rock. And then again 3-4 days after the rocks have soaked, and one more time in another 3-4 days. If the pH shows changes of more than .2 degrees, the rock will affect your aquarium water chemistry.
You might decide to use it anyway, however. Small shifts are balanced out by other factors like the presence of decaying plant matter and carbon dioxide. The larger volume of your aquarium may make a small amount of rock minerals insignificant. Regular water changes will remove these minerals whenever you add fresh aquarium water.
The second way is faster, however. Clean the surface of the rock as before to remove dust and dirt. Add a few drops of white vinegar and wait for a few minutes. The presence of bubbles or foam indicates that the acidic vinegar is reacting with calcium carbonate in the rock. Therefore, the rock will raise your tank’s pH.
How to Raise Calcium in Reef Tank?
Calcium is critical for reef tanks. Hard corals require it to create their reef-building skeletons. While soft corals use it for structural spicules that prop their bodies up. Calcium is added to most reef tank aquarium salt blends in high amounts. Maintaining a constant level of 450-500 ppm between water changes is difficult if you have a large coral population, however.
Many aquarists use crushed coral to raise pH but it’s less effective at raising calcium levels. Adding calcium to aquarium is best achieved through using aqueous solutions.
Bottled calcium like API Marine Calcium Reef Solution do a great job and are inexpensive. The main downside is that stable calcium levels are harder to maintain. Reef tanks are hard to keep because the ocean is a huge, stable environment. Changes occur over hundreds to thousands of years.
In a home aquarium, calcium concentrations will swing as corals use up what’s available. Redosing as calcium is used up will work but corals aren’t able to optimize their biochemistry well.
Reef tank Kalk Reactors are better for providing a slow and steady drip of calcium. They are also a source of alkalinity for your saltwater aquarium.
Conclusion
Sedimentary and metamorphic rocks that contain calcium carbonate will raise the pH in your aquarium. Avoid these and test any unknown rocks using vinegar or a soak in a bucket of water. Silicate-based rocks like granite and quartz won’t raise the water’s pH.
FAQs
Alkalinity will go up if you increase the carbonate content. Baking soda, or sodium carbonate, will do so. Dissolve one teaspoon per 10 gallons of water in a cup of aquarium water. Add it to your tank and re-rest your alkalinity 24 hours later. Extreme changes in alkalinity may be fatal to aquatic life. Add a quarter of this solution at a time if you’re unsure about the alkalinity level you want to reach.
Rocks like limestone are very effective at raising the pH of your tank. If you want a slight increase, use a single piece of limestone. A substrate of crushed limestone rocks or coral skeletons (aragonite) is even better at keeping the pH high (7.4-8.0).
