Using Limestone for Fish Tanks

Limestone is a popular decorative addition to aquariums, providing a natural, rugged look. But beyond aesthetics, limestone can also influence water chemistry in significant ways. This comprehensive guide will examine the key factors when using limestone in fish tanks.

We’ll explore limestone fundamentals, its effects on pH, safety considerations for fish and plants, where to source it, alternatives, and special considerations for ponds. With the right knowledge, aquarists can make informed decisions about using limestone to create a healthy, thriving aquascape.

limestone for fish tank

What is Limestone?

Limestone is a sedimentary rock primarily composed of calcium and magnesium carbonate. It forms over millions of years as marine organisms’ shells and skeletons accumulate on the seafloor and solidify into rock.

Limestone varies in hardness, texture, and color. Chalk, travertine, tufa, and marble are all forms of limestone. It’s one of the most abundant minerals, making up around 10% of all sedimentary rock.

In aquariums, limestone is popular for its textured, rocky appearance and neutral colors. It provides hiding spots and surfaces for beneficial bacteria to grow. Limestone’s porous nature also allows water to flow through tiny holes.

Beyond aesthetics, limestone can dissolve slowly and release minerals into the water over time. This can significantly impact your aquarium’s chemistry.

Is Limestone OK for Fish Tanks?

Limestone itself is non-toxic and safe for aquarium use. However, its ability to alter pH and water chemistry is unsuitable for all setups. You’ll need to consider:

  • The pH requirements of your fish – Limestone raises pH, so avoid it if you have soft water fish.
  • Impact on pH fluctuations – A sudden pH spike could shock fish. Gradual changes are safer.
  • Effect on water hardness – Released minerals increase general hardness (GH).
  • Tank inhabitants – Corals, invertebrates, and plants have specific needs.

Providing your fish and plants can tolerate higher pH and hardness, limestone is generally safe. But regular testing is essential to monitor changes and enable gradual acclimation.

Limestone in Different Aquarium Setups

Limestone can be used in freshwater tropical tanks, cold water setups, African cichlid aquariums, marine reef tanks and more. But suitability varies.

Freshwater Tropical Aquariums

Limestone works well for tropical species that prefer alkaline water between pH 7-8. This includes African cichlids, livebearers, and rift lake fish like Tropheus species. Start with a lower ratio of limestone and monitor.

Avoid for soft water fish like tetras, discus, and angels that prefer pH below 7. Limestone can shock them. Driftwood is a better choice.

Planted Aquariums

Limestone may not be ideal in planted tanks as it can reduce phosphate and carbonate hardness (KH) through chemical processes. These nutrients are vital for plant growth.

If using, provide plant fertilizers to replenish nutrients and maintain lush growth. Monitor for signs of deficiency.

African Cichlid Tanks

Limestone is a natural choice for replicating the rocky habitats of African rift lake cichlids. Texas holey rock is a popular limestone option.

Cichlids thrive in the alkaline conditions limestone provides. Aim for a pH around 8-8.2. Mix in aragonite sand for added buffering.

Marine Reef Tanks

Limestone raises pH and KH, helping buffer against pH swings in marine tanks. This provides stability for corals and invertebrates.

Use limestone rubble or larger rocks and monitor alkalinity. Avoid fine substrates like limestone sand which can rapidly alter chemistry.

Coldwater Aquariums

Cool water fish like goldfish and koi tolerate higher pH and benefit from the added minerals limestone provides. It offers shelter and beneficial bacteria too.

Limestone blends with the natural look of pond-style coldwater setups. Goldfish enjoy nibbling at the rockwork as well.

Impact on pH

One of limestone’s biggest effects is increasing pH and alkalinity. Let’s look at why it causes these changes:

Why Does Limestone Raise pH?

Limestone comprises calcium carbonate, which dissolves into free calcium and carbonate ions in water. The carbonate buffers and increases pH, making the water more alkaline.

Over time, limestone slowly dissolves and leeches calcium and carbonates into the water column. Testing alkalinity provides a proxy for these dissolved minerals.

Aragonite, crushed coral, and coral sand also influence pH similarly. The finer the particles, the faster it dissolves. Larger rocks release minerals more slowly.

How Much Will Limestone Raise pH?

There’s no universal pH increase amount, as many variables affect limestone’s impact:

  • Surface area – Smaller gravel or sand has more exposed surface to dissolve.
  • Water chemistry – Softer water with lower KH sees faster pH rises.
  • Aquarium size – Larger bodies dilute the effect. More impactful in small tanks.
  • Amount used – More limestone = higher pH potential. Start with 1-2 lbs per 10 gallons.
  • Flow rate – Good circulation spreads dissolved minerals faster.
  • Other materials – Driftwood, peat, and plants can lower pH.

It’s impossible to predict exact pH changes. Frequent testing lets you gauge the impact in your tank and make adjustments.

Keeping pH Stable

pH stability is crucial for fish health. Some ways to maintain steady pH with limestone include:

  • Adding limestone gradually rather than all at once
  • Providing ample biological filtration to process dissolved organics
  • Using limestone rubble rather than fine substrates
  • Mixing in materials like driftwood that release tannins to lower pH
  • Doing regular partial water changes to replenish mineral levels

Aim to keep pH changes below 0.2 over 24 hours. This prevents sudden swings that can harm fish.

Where to Buy Limestone

Limestone for aquariums comes in several forms suitable for different uses. Here are some options:

  • Texas holey rock – Light porous limestone with small holes, provides hiding spots.
  • Tufa rock – Extremely lightweight and filled with holes, allows water flow.
  • Limestone rubble – Irregular chunks and pieces create nooks and crevices.
  • Limestone gravel – Small pebble substrate 1-3 mm in size, dissolves faster.
  • Crushed coral – Fine 1-4 mm grains often used as substrate, quickly alters pH.
  • Coral sand – Super fine crushed coral under 1 mm for substrates. High pH impact.
  • Large limestone rocks – For stacking to build rock walls and structures. Releases minerals slowly.

Check at aquarium stores, garden centers, landscape suppliers or online. Ensure any limestone is safe, with no harmful copper or lead. Do an acid test if uncertain.

How to Test Limestone

Testing any new limestone before adding it to your tank is crucial. Some simple home tests can identify imposters:

Vinegar Test

  • Drop a few drops of vinegar onto a small, inconspicuous area of the rock.
  • It is genuine limestone reacting to the acetic acid if it fizzes or bubbles.
  • No reaction likely means it’s an inert rock, so it won’t alter pH.

Scratch Test

  • Use a knife or nail to scrape a small area of the rock.
  • True limestone will crumble somewhat when scratched.
  • Rocks that don’t flake apart are likely silicates with no pH effect.

Do these quick tests before fully submerging any new limestone in your tank. This avoids unintended chemistry impacts from lookalike rocks.

Unsafe Rocks and Alternatives

While limestone is generally safe, not all rocks are appropriate for aquarium use. Avoid:

Rocks Containing Heavy Metals

Copper, zinc, lead and other potential toxins can leach from rocks into the water, poisoning fish. Stick to inert aquarium-safe rocks.

Rocks That Alter Water Chemistry

Sudden major pH, GH, and KH spikes from fast dissolving rocks can shock fish. Examples are concrete, ironstone and quicklime.

Soft, Porous Rocks

Pumice, lignite and some sandstones lack enough density and can quickly crumble into fine particles that clog filters.

Salty Marine Rocks

Ocean rocks like basalt may release sodium and minerals not suited for freshwater. Collect rocks well above the high tide line only.

Here are some safe alternatives to limestone:

Lava Rock

Lava rock provides the rugged, rocky look without affecting pH or water chemistry. The holes and crevices make great hiding spots. Use in all aquarium types.

River Rock

Naturally smooth river rocks offer a gentle aesthetic. Their inert nature won’t impact water conditions. Available in a range of sizes.


Slate has an attractive gray appearance. It helps lower pH slightly but not drastically. Its smooth surface discourages algae growth.


Polished granite adds a sophisticated touch with neutral colors. Dense and non-porous, it won’t alter water chemistry.

Petrified Wood

The unique bark patterns create visual interest. Requires pre-soaking but won’t change pH. It provides shelter and enhances natural themes.

Dragon Stone

Dramatic ridges and valleys on dragon stone create striking aquascapes. It provides minerals but is less alkaline than limestone.


Releases tannins that gently lower pH. It provides hideaways and complements live plants. Boil first to remove excess tannins.

With many options, you can achieve the look of limestone without the chemistry effects by selecting rocks suited to your aquarium’s needs.

Special Considerations

Using Limestone in Outdoor Ponds

Limestone can also be used when constructing outdoor ponds. Limestone boulders blend into natural garden pond designs and provide shelter for fish.

Benefits include:

  • Adjusts pH for fish like koi and goldfish that thrive in alkaline water
  • Adds calcium needed for koi health and strong egg shells
  • Provides surfaces for nitrifying bacteria to grow

Use large limestone rocks rather than gravel to minimize rapid pH fluctuations from weather and rainfall. Test pH and KH regularly.

Removing Excess Limestone Buildup

Over time, limescale deposits can accumulate on limestone rocks, leaving white crusty patches. To remove:

  • Soak the rocks in a bucket of water with a limescale remover safe for aquariums
  • Use an old toothbrush to scrub off the buildup gently
  • Rinse thoroughly and return rocks to the tank

Vinegar or citric acid solutions also dissolve limescale safely. This restores the look of the rockwork.

Frequently Asked Questions

Can limestone harm my fish?

Limestone itself is non-toxic. But sudden spikes or drops in pH from limestone can stress fish. Acclimate fish slowly and monitor levels to prevent pH shock.

Is limestone good for live aquarium plants?

Limestone reduces key nutrients like phosphates and carbonates that plants need. It may not be the best choice for heavily planted tanks – use driftwood and root tabs instead.

Will adding limestone increase phosphates in my tank?

No, limestone doesn’t directly increase phosphates. It can lower phosphate levels through chemical binding processes. But low phosphates can starve plants.

What is the best rock for African cichlids?

Texas holey rock, a porous limestone, is an ideal choice. It raises pH, provides shelter, and mimics the rocky rift lake environment that African cichlids naturally live in.

Can I use limestone in a saltwater reef tank?

Yes, limestone can be used in marine tanks. It provides beneficial minerals and buffers pH changes caused by aquarium additives and calcifying corals. Start with small amounts and test calcium and alkalinity levels.


When selected carefully, limestone can make a stunning addition to freshwater and saltwater tanks. Its ability to raise pH and hardness requires monitoring, but it can create the perfect water parameters for fish that naturally thrive in alkaline waters. Pair with driftwood, plants, and rock alternatives to balance the chemistry effects.

Aquarists can create a healthy environment by understanding how limestone interacts with aquarium chemistry and selecting species suited to the resulting water conditions. Test frequently, acclimate fish slowly, and adjust as needed. With proper preparation, limestone can help create the aquatic landscape of your dreams.