As an aquarist, it’s imperative to know the physicochemical conditions of marine water to maintain coral reef tanks. Without calcium, proper and timely calcification can’t take place for the prolific growth of both LPS and SPS corals. Hence, if you want to breed colorful calcareous corals, you should know how often to dose calcium in reef tanks.

The required calcium dosage is directly dependent on the total calcium consumption of a reef system. And the total calcium consumption will directly affect the alkalinity levels of the tank water. So, by measuring the alkalinity, you can instantly figure out whether you should inject more calcium into the system to attain overall balance in the ecosphere.

Since calcium irrefutably affects the alkalinity and vice versa, it’s important to figure out where to draw the line. In today’s write-up, from dosage periods to dosage precautions to dosage parameters – let’s discover it all in one go.

How Often To Dose Calcium In Reef Tanks?

Benefits Of Calcium Dosing In Reef Tanks

Calcium is undoubtedly an integral part of marine ecology. But why’s it so necessary? And why do we have to separately introduce calcium ions into the system for proper sustenance?

Well, here are a few significant reasons why –

To Promote Calcification

Calcification is why coral reefs require additional calcium supplements for sustainable development.

In strictly biological terms, calcification refers to the amount of calcium deposition in tissue cells. Soft tissues gradually harden with time, thanks to the deposited calcium, and become teeth, bones, etc. in human beings.

In corals, calcium is the major component of the skeleton system as it forms and sustains the outer calcareous shells. Without any calcium in the water, the calcification process i.e. the formation of new exoskeletons, comes to an end.

Fun fact – the calcification rate doesn’t depend on the concentration of calcium in the tank water. As in, it doesn’t matter if the tank is loaded with calcium. The calcareous and scleractinian corals won’t magically develop new layers in hours due to the abundance of Ca2+ or Mg2+  in the water.

In fact, you’ll actually need to slowly adjust the calcium levels in the water. The concentration needs to go up in a linear and not in a quadratic manner.

For Mineral Growth

Calcium doesn’t only help to create new shells/skeletons for reef corals and similar invertebrates. It also helps to retain the structural integrity of the existing calcareous layers.

Calcium works as the primary nutritional element for corals. Without it, corals will start to lose their liveliness and eventually, become brittle enough to break apart easily.

Hence, reef tanks need ample calcium supplements to keep the circle of life ongoing for the calcareous inhabitants.

To Prevent Discoloration

The discoloration is usually the first sign that your corals haven’t been getting enough calcium as nutrients. As they keep losing their liveliness, their colors also start to change.

Even the most colorful corals start to portray an unappealing greyish tone over time. Once the calcium-alkalinity balance is restored within the tank, the lovely corals start to recover.

How Often To Dose Calcium In Reef Tanks?

So far, we’ve stumbled upon two holy grails in terms of info. One – You can’t restore the chemical balance by abruptly increasing the calcium density in the reef tanks or aquariums.

And two – You shouldn’t enforce calcium doses frequently without measuring the alkalinity levels of the tank water.

Then, how often to dose calcium in reef tanks?

Here are the factors you should consider for calcium doses –

Calcium Concentration

A sudden increase in calcium concentration can throw off the balance between alkalinity levels and calcium consumption. While it’s good to get habituated to a routine procedure, dumping more calcium during low consumption periods will increase the overall calcium concentration.

But don’t get scared. The damages from increased calcium concentration do not take place immediately. It’s more of a slow killer for calcareous invertebrates. Just like how too much of a good thing can end up killing you from the inside.

Bottom line is that – you’ll get many chances to reverse the conditions and restore the balance before the final blow. Admittedly, you should develop a habit of checking the calcium concentration of the water every two weeks or so.

That way, you can realize from the records if the concentration is low or high compared to the past weeks. Adjust the calcium doses accordingly to save your corals from malnutrition and overfeeding respectively.

Calcium Consumption

The calcium supplements you add to the water either get consumed or precipitated. Total calcium consumption refers to the ratio of used up calcium to added calcium.

The calcium consumption also determines the saturation situation of the tank water. When enough calcium isn’t consumed due to low buffering, the water becomes supersaturated due to the extra calcium added previously.

Alternatively, the supersaturated condition forces the alkalinity level of the water to go down significantly. Therefore, to ensure chemical equilibrium, the calcium doses should be decided based on the overall calcium consumption. The amount of calcium you add should effectively replace the amount of calcium consumed in the previous session.

Tank Temperature

Temperature, as always, is an automatic catalyst that accelerates or decelerates the chemical progression. Normally, the tank water of coral reef tanks isn’t very humid since the tanks mimic the cold marine regions.

But even then, sometimes temperature fluctuations do take place, especially during the summertime. Some corals then consume more calcium than anticipated. Calcium also precipitates faster due to temperature changes.

As such, the total calcium consumption goes up during these turbulent times. Needless to say, you too, will need to add more calcium than usual when this happens.

Alkalinity Levels

Corals consume calcium in the form of calcium carbonate. The alkalinity levels indicate the pH levels of the water. Alternatively, you can say that it indicates the range within which carbonate compounds get dissociated within the tank.

The dissociation constant fluctuates according to the buffering capacity of the tank water. Hence, when alkalinity is low, i.e. buffering capacity is low, less amount of calcium carbonate will get dissociated into carbonic acid in water.

Again, if the alkalinity is too high, calcium deposition will outperform calcium consumption – which is just as undesirable in coral reef tanks. And that’s why it’s important to figure out the alkalinity levels before dosing more calcium.

The aforementioned four factors help to determine the required amount of calcium doses. However, not everyone has the time or energy to keep track of the consumption, concentration, temperature, and alkalinity levels 24/7.

So, you can opt to leave the calculations to a calcium reactor instead or choose to trust the store-bought supplements. Simply follow the instructions to a T and add the necessary amount every 2-3 days, or as prescribed in the packages.

Additional Read:

Importance Of Calcium Reactors For Reef Tanks

In the previous segment, you found out about four key issues that influence the frequency of calcium doses. Calcium concentration, consumption, tank temperature, and lastly, alkalinity levels – these need to be measured periodically to ensure the presence of a viable amount of calcium in the water.

A calcium reactor is one such instrument that both measures and timely inserts calcium doses according to the results. Here’s an overview of the primary benefits you’ll avail of if you choose to invest in a chemical reactor –

Mineral Tracking

The calcium reactor detects the amount of calcium, bi-carbonate ions, magnesium, etc. in the sample water. Additionally, the reactor can also detect trace amounts of other softcore minerals inside the tank.

You can go through the results to easily figure out the current chemical conditions and alkalinity levels of the water. Calcium reactors, employing mineral tracking, help to keep the calcium input and output amount in equilibrium.

Calcium Control

A calcium reactor will automatically adjust the calcium doses to maintain the chemical equilibrium within the tanks. Calcium reactors dedicatedly promote the calcification procedure for the sustainable growth of the coral ecosystem.

Apart from calcium, they introduce carbonate into the system so the corals can receive proper nutrients. Carbon dioxide is added to a fixed amount of calcium within the reactor for the necessary carbonate compound formation.

According to the existing calcium levels, a controlled amount of prepared solution gets dumped into the water. This automatic calcium control also helps to restore the pH balance within the reef tanks.

How To Safely Dose Calcium In Reef Tanks?

Calcium reactors aren’t the only way to efficiently introduce calcium into reef tanks. So, if you don’t want to spend a fortune on high-end reactors, try adopting these methods –

2-Part Solutions

With 2-Part solutions, you’ll get to hit two birds with one stone. Or in this case, get two minerals for the price of one. 2-Part solutions include both calcium and magnesium, and hence, directly help to adjust the alkalinity levels of the tank.

In fact, they’re the best way to maintain the appropriate buffering capacity of water for proper calcareous growth. Furthermore, you’ll receive alkalinity promoters/buffers to increase the efficiency of the buffering capacity.

You can manually add the supplements according to the package instructions. For medium to large tanks, calculate the amount of calcium consumption by measuring the calcium levels for a few weeks.

Afterward, deduce the necessary amount in a way that the added amount complements the consumed amount.

And as far as the ‘how often’ question goes, there are actually no right answers. Some aquarists dose their tanks once or twice a week. Again, some use the supplements every day.

In reality, it eventually boils down to the overall calcium amount one way or the other. For instance – your tank could need 35 ppm of calcium every week for balanced conditions.

Now, you could break down the amount and dose 5 ppm every night instead of dosing 35 ppm at the end of the week. The end result won’t be that different.

Just make sure that the overall calcium level isn’t too far away from the 400-420 mg/L range. You can do regular calcium depletion tests to get the correct info on the current alkalinity conditions of the tank water.

Calcium Additives

As supplements, you can use calcium chloride, calcium hydroxide, etc. additives to promote carbonate production for further calcification. Between the two, calcium chloride is the frequently used additive for coral reefs.

Unlike the 2-Part solutions, these additives don’t usually include magnesium supplements alongside calcium. So, if you’re using additives like calcium chloride, remember to separately include magnesium supplements as well. Otherwise, the alkalinity levels will go down soon enough.

Also, don’t push forward both calcium and magnesium doses at the same time in reef tanks. Wait at least an hour before introducing magnesium into the system for better results.

The reason why it’s recommended to do so is that – the ions will precipitate faster due to the common ion effect if you add them together. So, let the calcium get settled first for uninterrupted carbonate compound formation. Afterward, introduce magnesium to adjust the alkalinity levels like a pro.


Lastly, you can adopt the Kalkwasser method, which is basically a variant of the additives. Here, you’ll be using calcium hydroxide instead of calcium chloride supplements.

Calcium chloride is great for reef tanks with SPS corals since the calcium demand isn’t that high. But with the Kalkwasser dosing, you can supersaturate the water to cater to LPS corals in large reef tanks and aquariums.

Kalkwasser is also great for tanks with low basicity as it can help to increase the pH levels of the water. Like calcium chloride supplements, you’ll need to separately add magnesium with Kalkwasser supplements as well.

Frequently Asked Questions

Q: What’s the ideal calcium level for coral reef tanks?

The ideal calcium level for coral reef tanks stays in the 400-420 mg/L range to mimic the ideal marine conditions. Depending on the size of the tank and overall calcium consumption, the calcium level can fluctuate accordingly.

Q: Will dosing calcium impact the DO level of reef tanks?

No, with the right calcium to water ratio, you don’t have to worry about the DO level going significantly up or down. However, without proper circulation, the calcium levels in a certain section might go up sometimes.

If the calcium density in a specific section becomes higher, then the DO levels there will go down. That’s why it’s important to ensure an all-around circulation while dosing.

Q: Can I get by without a calcium reactor for reef tanks?

With a calcium reactor, your precious reef tank will receive the proper all-around calcium circulation that it deserves. Alongside calcium, you can also trace the magnesium levels to predetermine the alkalinity situation before further steps.

However, they’re slightly expensive so they might not be the most cost-effective option for beginners. Especially if you’re managing a small-scale reef tank, then you don’t have to invest in a calcium reactor right away.

But once you switch to a larger tank, it’s recommended to reconsider this decision. Because calcium reactors will significantly decrease your workload by helping you determine the right Ca²+  dosage levels in a matter of minutes.

Q: Why is it important to use magnesium alongside calcium?

It’s important to use magnesium alongside calcium in reef tanks since they help to retain the alkalinity balance together. Calcium is often needed in heavy doses to cater to the calcium carbonate structures of coral reefs.

However, high levels of calcium will actually lower the alkalinity of the water itself – which will prove to be harmful to other aquatic beings in the tank over time.

Lower levels of magnesium also contribute to the aforementioned disturbances. Hence, it’s important to maintain a healthy ratio of magnesium alongside calcium to ensure proper alkalinity levels throughout the tank.

Final Word

So, how often to dose calcium in reef tanks for proper ecological sustenance? Thanks to today’s discussion, we discovered that the answer depends on four key components.

Calcium consumption, density, temperature, and alkalinity levels – you’ll have to stay vigilant about these four factors to know how often to provide calcium supplements.

Nowadays, the store-bought supplements come with the instructions that you should use them every 2-3 days.

However, for accurate dosage, there are no better alternatives to measuring the alkalinity levels. Without measuring the alkalinity levels, you won’t get a proper idea of the overall calcium consumption. If you keep blindly adding more calcium during low consumption periods, it won’t take too long for the entire ecological balance to go fully berserk.

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