Many aquatic enthusiasts believe that adding limestone in aquarium can be dangerous. But, that is nothing more than a fallacy. You see, there are many advantages to placing limestones into your fish aquarium.

But, the first order of business as an owner of fish is discovering the most fundamental detail about your pet — whether your fish belongs in saltwater or freshwater?

pH Level for Saltwater Fish and Freshwater Fish

You need to understand that the pH level is different for freshwater and saltwater fish, which is mainly why freshwater fish can’t survive in saltwater. And, the pH levels also affect the survival chances of saltwater fish in freshwater.

The pH level for freshwater fish needs to be as low as 5.5 to 7.5. On the other hand, the pH levels for saltwater fish need to be eight or above. If by chance, you have a variety of fish such as clownfish, dottyback, firefish, or damselfish, you should know that you have an aquarium full of saltwater fish.

And, your precious fish wouldn’t last long if the pH level of the aquarium were lower than what they need. But if you have a variety of goldfish, angelfish, guppies, and mollies, then you don’t need any limestone in your fish aquarium.

Limestone: Definition and Its Effect on the pH Levels of Water

Limestone is a sedimentary rock made of calcium carbonate (CaCO3). It is usually found in the form of calcite aragonite, and it may contain considerable amounts of magnesium carbonate as well. Now, limestone is a calcareous rock which means it contains calcium. And, it is used for many purposes, such as floor tiles, window sills, or sculptors.

But, the most significant advantage of limestone is that it isn’t rugged rock; it is widely available and very cheap. Now, you already know that it contains calcium and other trace minerals. And, these minerals are helpful not just for increasing the pH levels of water. You see, underwater plants thrive due to the presence of limestone too.

The pH level of limestone is 7, and it can react with the acid in the water and neutralize the acidity levels. Many pet parents of fish place limestone in their fish tank as limestone is very good at keeping the Ph level balanced inside an aquarium.

You see, if you try to change the Ph levels inside your fish tank too fast, you endanger the lives of your pet. But, the pH of limestone rock keeps alkaline levels higher and acid levels lower, along with reducing iron in the water. Moreover, limestone reduces the carbon dioxide in the water, making it a very healthy environment for your fish.

The Benefits of Limestone Rock for Aquarium

Limestone is gold for your pet’s aquarium, especially if dealing with saltwater fish. You see, there are several advantages to putting limestone in a fish tank, so let’s take an in-depth look at some of them:

– Aesthetically Pleasing

Placing limestone into an aquarium makes the fish tank look life-like and beautiful. The natural look of limestone adds to the charm of any environment, but underwater limestone creates a dreamlike appeal to your pet’s habitat. Furthermore, the natural holes in the limestone provide a ground for your fish to play a game of hide and seek.

– Inexpensive

Even if you have a large aquarium, adding limestone to the fish tank will not be a hefty weight on your budget.

– Cleans the Environment

Limestone is an alkaline agent, and placing limestone in a fish tank helps keep the algae from the water.

– Rocks That Lower pH

As mentioned earlier, limestone is an alkaline agent that works to increase the pH level of fish tank water. You see, limestone reacts with the acid in the water and neutralizes it. Hence, limestone is seen as the easiest way to harden aquarium water.

– Filters the Water

When you put limestone into your aquarium, the limestone can absorb the excessive salt and filter the water to make it the perfect living conditions for your saltwater fish.

– Helps in the Growth of Fish

Limestone contains many minerals, especially calcium and phosphorus. And these minerals help in the growth of fish.

– Removes Excess Iron

Acidic water contains a high amount of iron and its by-product. When you place limestone in an aquarium, you lower the Ph level of the water, automatically reducing the traces of iron and its by-product from the water.

If there is too much iron in the water, then the water can change color and odor. Such living conditions are incredibly harmful to fish, and you can quickly remedy the problem when you put some limestone into the tank.

Checking for Limestone in Your Fish Tank

You have to understand that many different varieties of limestone are available in the market. And, not all kinds are ideal to be placed underwater in a fish tank. But, it gets worse when you have rocks available in the market that look precisely like limestone but aren’t.

So, if you’re interested in getting the real deal, there is a simple solution to help you out.

If you walk into a pet store, you will find different rocks in a fish tank. Now, some are put in for a greater purpose, such as limestone to increase the pH level of water. At the same time, other rocks are merely ornamental.

So, if you are looking to increase the pH level of your fish tank, make the water cleaner, less salty, and your fish and the plant life healthier, you can put your rocks through a test. You can test the composition of rocks when you put a few drops of white vinegar on the rock.

If the vinegar starts to foam, then the rock is calcareous, and most calcareous rocks are limestone other than marble. And, it is very easy to tell the difference between marble and limestone.

Another means of testing whether the rock is limestone is by placing the rocks in a bucket full of water. You will have to monitor and test the pH levels, hardness, nitrate, and phosphate levels. If the water in the bucket increases in alkaline and reduces acidity, you know you have authentic limestone rocks.

Is Limestone Harmful for Fish?

The only time limestone can harm your fish is when you place limestone into the tank of freshwater fish.

You see, freshwater fish require lower pH levels. And limestone is known to naturally increase the pH levels of water inside fish tanks. So, make sure you understand the kind of fish you have in your tank and the pH levels your pets require before you put any limestone into the aquarium.

Limestone is a sure-shot way of increasing the pH levels of water in a fish tank. And altering the pH levels of water drastically can be more dangerous for fish than a higher acidity level in the water. So, yes, you should use limestone to increase pH levels in your aquarium. But, it would be best if you were very careful about the speed at which you introduce change inside your fish tank.

On the flip side, if you have saltwater fish that are not doing too well, placing limestone in most cases has proved beneficial.

Limestone naturally performs multiple tasks to improve the purity of water inside the fish tank. It filters water, removes algae, enhances plant growth, enhances fish development, reduces acidity, and even makes room for your fish to get some entertainment. Not to mention, all the while, your fish tank will look fab too.


We’re hoping that you’ll now see why limestone is so helpful in maintaining the pH levels of your fish tank. And, you are likely to excel at the maintenance and hygiene of your fish tank so long as you remember the following:

  • There is a difference between the pH levels for freshwater fish and saltwater fish.
  • Freshwater fish need to have a pH level of 5.5 to 7.5 in the water.
  • Saltwater fish need to have more pH levels in the water. The pH level for saltwater fish needs to be at least eight or above.
  • Limestone helps harden the water and increases the pH level of water.
  • Many fish owners use limestone in their fish tanks for decoration.
  • Limestone provides nutrients to the fish and plant life of a fish tank.
  • It can also filter the water by absorbing the excess salt.
  • Limestone is also used to remove excess iron from water in fish tanks.
  • Adding limestone in an aquarium will help clean up the water as limestone removes algae from the water.
  • Remember that limestone should only be placed in a fish tank where the high acidity levels and soft water harm the fish.
  • Limestone also provides a perfect play area for fish where they can entertain themselves through hide and seek.
  • Limestones can reduce carbon dioxide in the water, making the aquarium water levels more suitable for improved fish health.
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