Skip to Content

12 Easy Ways To Add Calcium To Freshwater Aquarium

Calcium is an essential component of freshwater aquariums because it keeps fish and other species strong and healthy. Fish crave calcium for bone growth and egg development, and higher calcium levels in a fish tank can also promote longer lifespans.

Your freshwater aquarium’s calcium content can signal a healthy tank or lead to fish death, in severe cases. However, it doesn’t have to be so challenging to control. This article will discuss the many ways you can add calcium to a fish tank, why calcium is important for an aquarium, and how it interacts with pH.

12 Easy Ways To Add Calcium to a Freshwater Aquarium

how to add calcium to freshwater aquarium

There are several ways that you can add calcium to a freshwater aquarium, such as adding cuttlebones, calcium blocks, calcium hydroxide (kalkwasser), liquid calcium chloride, and wonder shells. Additionally, you can improve calcium levels in aquarium by using crushed oyster and eggshells.

Calcium reactors, two-part solutions, and dietary supplements can also be effective options for improving calcium in aquarium. Remember to perform regular water changes to ensure the maintenance of adequate calcium levels in your aquarium.

Want to learn in detail? Keep Reading!

Do Frequent Water Changes

how to add calcium to freshwater aquarium
Depending on the hardness of the tap water in your area, you may be able to get enough calcium in your water tank simply by changing the water often enough. Ideally, you should be changing 25% of the water every two weeks. Be careful to test your calcium levels before relying on this method, as many tap water sources don’t have enough hardness for a fish tank.

If you’re not sure whether your water is hard or soft, consider the following signs of a hard water supply:

  • Spotty and filmy dishes
  • Soap won’t lather
  • Tubs and showers are scummy and hard to clean
  • Skin is dry and squeaky in the shower
  • White crust around drains, showerheads, and faucets

You can also call your local water municipality, who’d be able to tell you what the mineral levels in your water supply are.

Another option is to buy something like this USA Made Total Hardness Test Strips from a home improvement store, which will tell you what’s in your water in detail.

Use Cuttlebones

how to add calcium to freshwater aquarium
Cuttlebones are larger interior shells harvested from cuttlefish and contain about 85% calcium. These are a great choice for adding calcium because they’re not only full of the mineral but also work as a decoration. Cuttlebones are also inexpensive, widely available, and dissolve very slowly for an even and steady release of calcium.

For these reasons, cuttlebones are among the best ways to add calcium to your fish tank steadily. That said, it can produce a foul odor if left to decay for too long.

Cuttlebones are very large, so you might want to start with just a small, broken-off portion of the shell about one inch in length. These can also float, so consider weighing down the bone with pebbles or even boiling it first.

Add Egg Shells

how to add calcium to freshwater aquarium
Another way to add calcium to your fish tank is by adding eggshells. Be aware that fish can make a mess of eggshells, however, so it’s best to put them someplace where the fish can’t access them. Or, you can grind them into a powder so that they sink to the bottom of the tank.

Ideally, you’d add boiled eggshells with their membrane removed. Try to remove eggshells from the aquarium once they’re no longer adding calcium to the water because they can create a foul smell if left too long.

Add Corals

how to add calcium to freshwater aquarium

You can decorate your aquarium and add calcium at the same time by adding corals to your tank. This could be whole live corals or a ground coral substrate that lines the bottom of the tank. Ground coral has the added benefit of giving the tank a natural look; an unnatural reflective glass bottom can be disorienting for a fish.

Corals have so much calcium because of their stony exoskeletons, secreted both as a self-protection method and as a way to attach to a rock surface.

Use Calcium Blocks

how to add calcium to freshwater aquarium
Calcium blocks are concentrated calcium pressed into a block form and dissolve over time into the water. They last for about two weeks at a time and also tend to include magnesium. Compared to cuttlebones, these blocks dissolve faster and have a more immediate impact on the tank’s calcium levels.

Add Crushed Oyster Shells

how to add calcium to freshwater aquarium
Crushed oyster shells have high amounts of calcium and have a significant amount of salt content and should be used sparingly in a freshwater aquarium. However, they do make for an efficient and attractive way to add calcium to your fish tank.

Add Wonder Shells

how to add calcium to freshwater aquarium
Wonder shells like the Weco Wonder Shell last for about a month and clarify the water while raising calcium levels at the same time. They’re typically cleaner and easier to use than coral, even though they’re artificially made. Because they’re so concentrated, it’s best to start with small wonder shells one at a time.

Use Liquid Calcium Chloride

how to add calcium to freshwater aquarium
Liquid calcium chloride additives like the Brightwell Aquatics Calcion additive are available at most pet stores and can be an efficient way to add calcium to your tank. However, they can be costly to use, so these are best for tanks with a lower calcium demand. These can also cause a drop in alkalinity over time, lowering the water’s pH balance.

Liquid additives can either be dissolved all at once directly into the water on a regular schedule or implemented via a drip system. To implement a drip system, follow these steps:

  1. Collect your supplies: an 8 oz (237 mL) water bottle, rigid tubing, flexible tubing, an airline tubing valve, and a drill. The valve should be a plastic one, as the metal will erode.
  2. Drill two holes in the lid of the water bottle. These holes should be just wide enough to allow the rigid tubing to fit in place.
  3. Cut a piece of the rigid tubing at three-fourths the height of the water bottle.
  4. Place the piece of tubing in one of the holes in the lid.
  5. Attach the flexible tubing to the outer end of the rigid tubing. The flexible tubing length should be enough to reach from the bottle to the sump or aquarium.
  6. Attach the airline valve somewhere along with this piece of flexible tubing.
  7. Attach another four-inch (10.16-cm) piece of flexible tubing into the other hole in the water bottle lid.
  8. Fill the water bottle with a solution of calcium chloride dissolved in aquarium water.
  9. Screw the lid onto the bottle and close tightly.
  10. Open the valve and blow into the four-inch (10.16-cm) tubing length until water starts dripping from the longer tube.
  11. Close the valve, then position it in the sump.
  12. Adjust the dial for your desired drip rate.

The correct drip rate will depend on your tank’s size, the concentration of calcium chloride you’re adding, and the amount and species of fish you have. You should regularly test the levels of calcium, as well as the pH and alkalinity. Too much calcium chloride can make the water acidic and over-concentrated with minerals.

Add Kalkwasser (Calcium Hydroxide)

how to add calcium to freshwater aquarium
Kalkwasser, German for “limewater,” is a liquid made by combining calcium hydroxide powder and reverse osmosis (RO) water or water with a high pH. Kalkwasser has the added benefit of increasing the pH balance to an appropriate level while also increasing the calcium levels.

It is also an inexpensive solution that doesn’t leave residue or create a foul smell, which is a favorite choice of many aquarium owners. It’s easy to prepare with its just-add-water design and contains only the ions that you need.

One caveat is that kalkwasser powder is dangerous to inhale because it’s a caustict substance. It also needs to be used the day that it’s mixed, or else it may interact with atmospheric carbon dioxide and become unable to dissolve in the aquarium water.

Use Two-Part Solutions

how to add calcium to freshwater aquarium
Two-part solutions are chemical solutions that balance both the calcium levels and the pH simultaneously without altering the water’s chemistry. An example is the Two Little Fishies’ C-Balance solutions, sold in two half-gallon (1.89-liter) containers, one with solution A and the other with solution B. The idea is that you first add solution A to the water and then solution B.

Consider Calcium Reactors

how to add calcium to freshwater aquarium
A calcium reactor has the following parts:

  • Calcium carbonate or aragonite media dissolves in the water to release calcium. Different media types will require different pH levels to dissolve.
  • Carbon dioxide, which changes the pH levels to promote or discourage dissolution. Controlling the carbon dioxide flow rate allows you to control how much and how quickly the media will dissolve.
  • The solenoid valve opens and closes in response to electricity, giving you control over the flow of CO2.
  • A pump pushes water from the aquarium through the reactor.

Calcium reactors control calcium levels along with alkalinity and magnesium levels. A calcium reactor sits alongside the sump underneath the tank, taking in carbon dioxide gas from a tank and dissolving an interior piece of aragonite or carbonate to release minerals.

How To Fine-Tune the Carbon Dioxide Flow & pH?

You can adjust the rate of calcium increase by increasing or decreasing the flow rate of carbon dioxide into the reactor, which alters the pH. Follow these steps to fine-tune the flow of carbon dioxide and the pH appropriately:

  1. Set the reactor at a low flow rate, like 40 drips per minute. This equates to about ten bubbles of CO2 per minute.
  2. Adjust until the pH is about 6.5 to 6.7, measuring with a test kit or probe. If the pH is too high, turn down the flow rate. If the pH is too low, turn up the flow rate. Allow the reactor a few hours to respond to changes after you make them.
  3. Monitor the tank’s calcium levels to make sure the reactor is supplying enough for the level of absorption of the species in the tank.
  4. Once you’ve found a flow rate that works, check the tank’s levels every few weeks.

Ensure that you have good air and water circulation in your tank, or else excess CO2 can build up and lower the tank’s pH too much. If this does happen, you can top off your tank with a bit of kalkwasser to restore the balance.

A calcium reactor needs very little maintenance once it’s set up and is a great choice for large tanks with significant calcium demands. However, the initial setup is expensive and complicated, so it takes some investment.

See the following video for more information about how to set up your calcium reactor:

Supply Dietary Supplements

how to add calcium to freshwater aquarium
If you have snails in your tank, you can consider feeding them tums or other antacid tablets as a way of getting enough calcium into their systems. These can change the color of the water once dissolved but are an effective and efficient solution for low calcium environments.

For fish, many flake foods come with mineral supplements, including calcium and phosphorus. You can also find mineral supplements in bone or meat meals, available for purchase from most pet stores.

Why Do Aquariums Need Calcium?

Natural water sources like lakes and rivers have calcium deposits because of the erosion of stones. Nearby plants, invertebrates, and fish absorb the calcium and use the nutrient for growth and development.

The concentration of calcium in natural freshwater sources is anywhere between 0.5 milligrams per liter and 75 milligrams per liter, depending on the landscape. The American Midwest sees calcium concentrations of about 20 milligrams per liter (0.003 oz per gallon) in most lakes. Meanwhile, marl lakes in areas with high limestone content have much higher calcium levels.

The lowest freshwater calcium concentrations are found in the boreal zones of Canada and Europe, where the levels are about 5 milligrams per liter (0.0007 oz per gallon). Lakes at and below this calcium level aren’t suitable for many invertebrate life forms, who rely on calcium absorption for building their exoskeletons.

Why Fish Need Calcium?

Fish rely on calcium to build their skeletons, make eggs, and absorb calcium through their gills. Still, they can also take in enough calcium through their diets and are less dependent on dissolved calcium than their invertebrate relatives. For them, an appropriate calcium level has more to do with matching the hardness of their native environments.

When a fish experiences a sudden change in calcium concentration, it can become very sick and lethargic or even die. For this reason alone, you’ll want to make sure that you know two things: the approximate calcium concentration in the fish’s native water source and how the pet store or pet supplier held the fish.

Calcium-rich waters are also suitable for fish because not all fish food is fortified with enough mineral content. And because aquariums don’t have natural calcium sources, you need to supplement them with artificial calcium deposits for your fish and other species to grow properly and live a long life.

Calcium additives are especially important if you have a freshwater aquarium, which has less natural water hardness than a saltwater aquarium. Water hardness simply refers to the amount of calcium and magnesium dissolved in the water and is important to the growth and development of many fish species.

How Often Do You Add Calcium to an Aquarium?

how to add calcium to freshwater aquarium
Your freshwater aquarium should have a calcium hardness of about 70 to 90 mg/L (0.009 to 0.012 oz/gal), which you can check with a calcium testing kit like the Aqua Care Pro testing strip set. This allows you to quickly and accurately determine whether it’s time to change or add more calcium additives.

You can tell that you have too much calcium in your fish tank if scaling starts to appear on the edge of the water, but there isn’t a quick visual way to see that you have too little calcium in the tank; you really need to find a calcium testing kit.

Usually, you’ll need to add more calcium every couple of days. However, this varies according to the calcium demand of the species you have in your tank, so you should always test the calcium levels in your aquarium before you add more.

Different sources of calcium have different concentrations. This means that where you might need to add just a small amount of kalkwater, you’d need a large piece of cuttlebone to reach the same level of calcium.

Remember that you can combine different methods of adding calcium to the water depending on your needs. You can use one primary source of calcium, like slow-releasing ground coral, and then add a liquid additive like calcium chloride as needed to balance the mineral levels. Just be careful not to add too much because this can cause severe problems and even death.

What Happens if You Add Too Much Calcium to an Aquarium?

Although you can get away with including a large piece of calcium-rich substance in your aquarium, you need to make sure that it’s dissolving at an appropriate rate so that it doesn’t overwhelm the tank with too much.

One sign that you have too much calcium in your tank is that it starts scaling or solidifying on the edges of the tank. This can cause major problems for your fish and can clog your water filters.

Most fish and other aquatic species can survive outside their ideal range of calcium concentrations but have difficulty handling sudden water chemistry changes. These sudden changes can cause stress, lethargy, erratic swimming, growth problems, and even death.

You should be especially careful about adding new fish to your aquarium and making sure that your tank’s water chemistry matches the environment the new fish is coming from.

What About pH?

how to add calcium to freshwater aquarium

The term pH refers to the number of hydrogen and hydroxide ions in the water. Normal water has an equal balance of both, resulting in a pH value of 7.0 on a scale of 14.0. Acidic solutions have a lower pH than normal water, and alkaline or basic solutions have a pH higher than 7.0.

Different fish are accustomed to different pH levels according to the water sources where they come from. Saltwater fish tend to thrive in water with a higher, more basic pH, where freshwater fish tend to prefer a lower, more acidic pH. The right pH for a freshwater tank is usually between 5.5 and 7.5, but you should be sure to check the right pH for your species.

The following are recommendations for water pH for different freshwater fish species:

  • Angelfish: 6.5 – 7.0
  • Clown Loach: 6.0 – 6.5
  • Goldfish: 7.0 – 7.5
  • Harlequin Rasbora: 6.0 – 6.5
  • Hatchetfish: 6.0 – 7.0
  • Neon Tetra: 5.8 – 6.2
  • Plecostomus: 5.0 – 7.0
  • Silver Dollar: 6.0 – 7.0
  • Tiger Barb: 6.0 – 6.5
  • Zebra: 6.5 – 7.0

Like with calcium levels, the most important thing for your fish is to avoid sudden water pH changes. You should know what the pH of the fish’s source water is and make sure that the different fish you’re keeping have compatible needs. Taking your fish home from a pet store aquarium with a different pH than your home aquarium is a leading cause of early fish death.

How To Adjust pH Levels in a Fish Tank?

Your fish tank’s pH should be tested about every two weeks or at least once a month. You can then raise or lower your tank’s pH with the following techniques:

  • To raise the pH of your tank, add crushed coral or baking soda. If you’re adding baking soda, dissolve it in water and remove the fish before adding it to the tank.
  • To lower the pH of your tank, add peat moss or driftwood to the tank, or decrease the aquarium’s aeration.

The pH level of an aquarium can be pushed out of balance by calcium additives, as well as some other factors listed below:

Remember that stability is the main goal and that you should avoid any unintentional, major pH changes.

How Calcium Levels and pH Are Related?

When an aquarium has a low pH, this means that the water is more acidic and will more readily dissolve calcium-rich mineral substances like cuttlebones and oyster shells. This means that a low pH will increase the amount of calcium in the water, provided that there’s a source of calcium there to be dissolved.

When calcium dissolves into the water, it acts as a buffer, preventing the pH from drastically changing, even with the addition of a strong acid or base. This means that as the pH begins to drop, the calcium will dissolve and cause the pH to stabilize.

However, if the pH is too high, the calcium will not dissolve, and therefore will not act as a buffer preventing the water from becoming too basic. This results in water that is too low in calcium and too high in ammonium ions, leading to the development of ammonia, which is toxic to fish and can result in death.

That’s why it’s important to measure both the pH of your water and the calcium levels: because they’re related and can have serious consequences if not handled properly.

Natural Water Sources and pH

Different water sources have different ideal pH levels depending on the needs of the species that live in that habitat. However, all water sources can be thrown off-balance by the same few factors listed below.

  • Wastewater is the number one factor that alters the pH of wetlands in the United States. This refers to any water that altered by humans, including pool water and sewage water, as well as storm drainage. Adding or removing chemicals to water — even in the process of waste treatment — can negatively impact the pH of wetland waters.
  • Mining, construction, and industrial operations can result in unnatural mineral deposits that upset the soil’s pH balance, in turn upsetting the pH balance of the surrounding wetlands. For example, a mined mineral like diabase can increase the pH of a wetland when brought to the surface.
  • Acid rain happens when sulfur dioxide and nitrogen oxides are released into the atmosphere, usually through fossil fuel burning. This decreases the pH of natural water sources to a dangerous degree. Acid rain can also be blown long distances and impact environments a long way away from the source.


The best ways to add calcium to an aquarium depend on your personal preference. Still, many of the more cost-effective ways involve adding calcium-rich shells or bones to the water and monitoring the pH to make sure it’s dissolving at an appropriate rate. Liquid additives are also popular, as well as kalkwasser, made from a powder.

You might also be interested in: