How To Make Aquarium Water Harder – 5 Proven Ways To Do It

Last Updated on March 6, 2022 by Marco C.

Wondering how to make aquarium water harder? These are proven and safe ways to improve the quality of life of your aquatic pets.

Water hardness is an important water parameter that makes your aquariums a safe and healthy place to live. Knowing and understanding what it affects and how it’s affected gives you control over your entire underwater world.

In nature, aquatic animals are adapted to the chemistry of their domestic waters, but sometimes the hobbyist’s tap water does not match these conditions, and as a result, the animals suffer.

Stay with us and learn how to make aquarium water harder.

What Is Water Hardness?

In scientific terms, water hardness is a measurement of dissolved calcium and magnesium in water. On the other hand, in layman’s terms, you can recognize the hardness of water as that slimy feeling after washing your hands with soap and water or as that white residue that makes your drinking glasses dirty (less than crystal clear).

The importance of general water hardness is that it is linked to water pH. Therefore, if you are having a problem with your pH level and either need to raise or lower it, however, you will first need to adjust the hardness.

As we mentioned, it measures the number of minerals in the water, in particular, calcium and magnesium ions. These minerals are very important for many fish and invertebrates because they help build bones and exoskeletons, regulate metabolism as well as facilitate ion exchange. If the parameters are not good, i.e. if you keep your pets outside their optimal hardness range, this can result in them not growing as big or not living as long.

How To Make Aquarium Water Harder

Water Changes

Good water parameters are the key to the health of your aquatic pets and plants. Since most hobbyists use tap water, it is necessary to determine whether it is hard or soft. If it is moderately hard or very close to extreme hardness, you just don’t have to go any further.

Changing tap water more often in most cases will be enough to make up for the general hardness depleted by the fish and plants in your fish tank.

Water Changes

Learn more about: How to Make Tap Water Safe for Fish?

Crushed Coral & Limestone

Another method of making aquarium water harder involves adding crushed corals and limestone to your tank. You can add them to an area with high water flow, such as an aquarium filter.

Crushed corals originate from dead coral reefs and are mostly composed of calcium carbonate. Due to their composition, they slowly release calcium and carbonate into the water when added to your tank while raising the hardness of the water.

Similar to corals, limestone is added to the main aquarium display or to the filter to help raise the hardness. Since it releases minerals into the water, it also causes the pH to rise respectively.


Another answer to the question “how to make aquarium water harder” is aragonite. Aragonite contains the most soluble form of calcium carbonate and is available in the form of rocks or sand.

Oolitic aragonite is spherical in form (carved out from tropical seas) and is most recognizable by the fact that it is liquidized with a little flow. Many brands mix it together with crushed coral in one product to make it more visually appealing. Also, aragonite is mostly used as a substrate for hard water-loving fish, it is much more pleasing to the eye than crushed coral.

Water Remineralizer

By the process of demineralization, water is deprived of beneficial mineral contents. The process itself has a direct connection to the reverse osmosis system that helps soften the water.

On the other hand, a water remineralize replenishes your soft water with those missing minerals. This primarily refers to calcium and magnesium.

When it comes to adding to your aquarium, the process is quite simple. Just get a post-filter that is garnished with some form of magnesium or calcium, preferably calcite or corosex. Calcite contains calcium carbonate, and corosex contains a magnesium compound. You can even use a post-filter that contains both compounds; this would be ideal, but not necessary.

After purchasing a filter simply run demineralized water through it, keep in mind that the water must be fresh. That would be it if you follow these instructions you can surely achieve your goal.

Read more about: How To Lower KH In Aquarium &; 5 Best Tips And Tricks

In Conclusion

The whole story about water hardness can be confusing, but it is essential for maintaining a stable and safe environment for your aquatic pets.

As you know, different aquatic animals need different levels of total hardness depending on where they originate from in the world. Accordingly, these levels will need to be matched in your fish tank by adjusting as needed.

Conduct general hardness, carbonate hardness, and pH testing on a regular basis to make sure you provide your pets with everything they need for a long and healthy life.

That would be it, now you know how to make aquarium water harder. If you have any questions don’t hesitate to leave a comment below!

Read more about Saltwater Aquarium Kit For Beginners – 10 Most Important Things You Need To Know


How Do You Harden Water?

You can achieve this by adding calcium carbonate (CaCO3) to your tank. Simply add two teaspoons per 50 Liters of water, thus you should increase the hardness of the water by about 4 dH.

How Do I Fix Soft Water In My Aquarium?

If you want to raise the hardness of your aquarium water you can use methods such as changing the water or adding some sea shells, aragonite, coral, limestone, marble chips, etc.

Does Aquarium Salt Increase Hardness?

Although aquarium salt is often suggested as a good way to increase hardness, this is unfortunately not entirely true. It will, but in a very small, almost insignificant percentage.

On the other hand, a mixture of sea salt has proven to be much more effective, but it significantly raises the salinity of the water, which most of your freshwater fish will not appreciate. Most freshwater animals are susceptible to elevated levels of salinity, and such levels have been identified as one of the factors responsible for the development of the disease known as Malawi Bloat.

How Do You Increase Water Hardness Without Increasing Ph?

You can do this using crushed coral, limestone, dechlorinated water, potassium bicarbonate, or alkalinity buffers.

In case you are afraid that raising the water hardness will have a direct impact on raising the pH level, simply wait until the pH level is at the bottom of the safe range.