Reef Tank Calcium Level

Image Credit: 915Mang via YouTube
Image Credit: 915Mang via YouTube

Reef Tank Calcium Level


Natural seawater has very specific water chemistry that marine life has adapted its biology to, so we as aquarists must do everything we can to match those natural levels.

Marine life needs very specific conditions to thrive. For instance, corals will use calcium to help build their skeletons when it is at the appropriate levels. Low levels of calcium will prevent the healthy growth of your marine life.

Why low calcium can be an issue for your organism’s health and why testing is one of the most important things you can do regularly.

This article will help inform you how to best match the marine environment and the chemistry of natural seawater a little better. We will answer why low calcium can be an issue for your organism’s health and why testing is one of the most important things you can do regularly.

Best Reef Tank Calcium Level

380ppm – 450ppm is the most appropriate range to keep calcium levels within for reef aquariums.

Lower calcium levels can be too low for corals to properly grow healthy skeletal structures.

Higher calcium levels can negatively affect alkalinity and pH levels.

Do I need to add calcium to my saltwater tank?

Yes, you will most certainly need to add calcium into your tank at some point. Even if you are running a calcium reactor, the machine will require that you maintain its concentrated solution over time as it doesn’t create calcium out of thin air.

Aquariums need a stable balance of calcium to keep a healthy environment for fish and coral. It also helps to maintain the levels of alkalinity, which in turn aid in the balance of the pH levels in the tank. Everything is connected in the aquarium environment, after all.

Can calcium be too high in a marine tank?

As beneficial as calcium can be for coral health and growth, there is a delicate balance that must be upheld in your aquarium. We have discussed in the past how almost every element in a reef aquarium is connected to another element. Changing one thing can lead to a ripple effect of consequences if you are new to the hobby.

In calcium’s case, it is connected with alkalinity levels. As calcium rises, alkalinity levels tend to drop. At the same time, an increase in alkalinity can decrease calcium. If your calcium levels are too high in your reef aquarium, you run the risk of your alkalinity is too low.

The problem with this is that it affects the buffering capabilities of the water, which in turn affects the pH levels.

High calcium levels can even stress your invertebrates out. You might notice some of these behaviors in your organisms like erratic swimming from your fish.

Do all corals use calcium?

All corals need adequate calcium levels to some extent. That is because corals use calcium to create the supports in their skeleton and promote further growth. If the corals do not have calcium (or have too much), their growth could be stunted or damaged.

Soft corals will need some calcium to be healthy.  The amount of calcium a coral does need will depend on the species, but here are some general guidelines for soft corals and calcium levels.

Generally, you should have between 380 and 450 ppm of calcium in your aquarium, but with soft corals, you can be on the lower end of this spectrum since they will not need as much as a hard coral. When in doubt, aim for the middle with 415 ppm and adjust when needed.

How to lower calcium in reef tanks

Due to the delicate balance between calcium and alkalinity, there might be times where your will need to lower the calcium levels in your reef aquarium.

Water changes

Performing a water change as little as 15-25 percent can help to reduce the level of calcium in your tank significantly.

However, pay attention to the calcium in the salt mix that you use for your water. You want to ensure that you provide a stable salt mix to the water and add too much. Salinity is important, but as long as you are not changing too much, a small discrepancy in salt won’t be too bad.

You also want to be careful that you are not lowering the water parameters in your tank for other elements like alkalinity or phosphate. You should have a few test kits available to test all the parameters your tank needs to thrive so that you can ensure that when you fixed the problem, no additional problems spring up.

Testing is an important step when you have finally completed the change; if you are noticing that nothing has changed, there is a chance that the salt mix that you used was too potent. If you noticed a massive dip, try to raise it with one of the methods listed below.

Stop adding Calcium

This might sound obvious, but there are two areas where the ritual might have clouded your judgment. If you are double-dosing your tank with alkalinity, just hold back and add only the alkalinity. Since calcium and alkalinity have a give and take relationship, the calcium levels will begin to drop as alkalinity increases. Use a regular test kit to ensure that you don’t overshoot your target range.


If you don’t want to mess with the routine of your aquarium too much, you can try and wait for the levels to stabilize. However, this method can take a few weeks to work as the corals in your take in the calcium. This method should also only be attempted if your calcium levels aren’t higher than 550 ppm.

How to raise calcium in reef tanks

There are not many ways that you simply raise your calcium levels immediately; however, there are ways to ensure that you have a constant supply of it.

  • Supplement – There are supplements on the market that allow aquarists to give specific trace elements a boost with a few well-measured scoops. Just remember that alkalinity is affected directly by a change in calcium. Often, the two-dose method is preferred due to the dynamic. You could also consider using calcium nitrate as an additive as it is a great balance between alkalinity and calcium.
  • Two-Part dosing – Two-part dosing is the most common way to maintain calcium and alkalinity levels in an aquarium since both can receive a boost, so neither will fall drastically.
  • Calcium Reactor – A calcium reactor is an expensive but consistent way to create a stable calcium level in your aquarium—a great thing for especially large reef aquariums.

How do you test calcium in a reef aquarium?

Measuring your calcium levels is just as important as measuring any other element in your tank, like temperature or pH.

That is because calcium is not only beneficial to the growth of corals but calcium and alkalinity have a delicate relationship that requires the two to be on good terms with one another. In fact, you should make it a practice to measure both calcium and alkalinity at the same time due to their back and forth tendencies.

Alkalinity should remain between 2.5-4 mEq/l for a coral environment, but You will need to adjust for the difference calcium makes in the alkalinity mEq/l as it will begin to plunge if not supplemented correctly. However, alkalinity can also be measured in dKH. If that is the case, aim between 8-11 dKH.

The best way to measure the calcium levels in reef aquariums is to buy a test kit. There are a lot of different test kits available through different commercial retailers.

As much as calcium is connected with alkalinity and ph, it should be noted that it does affect other factors in the environment. Magnesium, for example, is another variable that has a close relationship with calcium. Your magnesium levels should be 3 times that of the calcium in the environment, so about 1200 ppm.

Magnesium has a direct correlation with how well organisms will be able to take in calcium in the first place. When you are testing for calcium deficiencies, make sure you are testing your magnesium concentration.

How much calcium does an aquarium need?

Ensuring that you have the correct level of calcium in your aquarium will help in proper coral growth. Most reef aquariums should aim for a calcium level between 400 and 450 ppm to ensure healthy growth.

Reef tank calcium dosing

As we have discussed, your aquarium should maintain a calcium range between 400 and 450 ppm. The most common way reefers accomplish this range is by two-part dosing with calcium and alkalinity. These bottles are sold at most retailers and come with measurement instructions for tanks.

However, you should start out with the minimum recommendations and work your way upwards until you’re safely within the preferred range for calcium/alkalinity. Wait around 10 minutes between each dose and test 24 hours later.

Does adding calcium raise pH?

In short, yes, calcium can increase the pH level in your aquarium. One of the best ways to increase the pH in an aquarium is to provide a seashell substrate that is made up of calcium-rich shells.

These shells will help to increase the pH over time.

How do I maintain calcium in my marine tank?

The best way to ensure a stable calcium level is either to invest in a calcium reactor or to implement regular two-part dosing and testing into your care routine.

These practices will ensure that you are keeping a stable environment, but they require that you are staying on top of your aquarium’s care.

Do you need a calcium reactor for a saltwater tank?

Calcium reactors are an expensive but reliable piece of aquarium equipment that can make maintaining your calcium levels a breeze. However, some people might view it as an unnecessary luxury simply because an aquarist can easily implement two-part dosing and supplements into their aquarium care routine without spending the money on the reactor.

That being said, larger aquariums can be even more difficult to ensure equal care for, and the fewer factors you need to think about, the better you may be.

Calcium reactors can make maintaining the needs of the larger aquarium much more manageable and allow you to focus on other more important factors. They use a higher concentration of calcium carbonate to supplement the needs of your tank.

The reactor uses calcium carbonate media dissolved to make calcium and bicarbonate distributed throughout the water. Or, to put it more simply, it is like the machine is reversing the process of calcification.

Carbon dioxide is used to introduce nitric acid into the reactor to dissolve the calcium carbonate to create the new ions. You should still carry out regular testing to ensure that your reactor is working properly.

Final thoughts

Understanding the importance of calcium to your tank’s health will help to ensure that your organisms are thriving in a safe and stable environment. Marine life has evolved to survive in very specific water chemistry, so it is our jobs as aquarists to try our best to match those natural conditions found in the ocean.

The most important thing you can do is to perform regular testing of your marine tank. This will keep you in the know at all times of how closely your aquarium is matching natural levels and will allow you to act according.

Water changes are a double-edged sword, so whenever you are trying to correct one problem like a low calcium or alkalinity level, you should test for other variables besides those after the water change.

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