Reef Tank Alkalinity (How To Raise, Lower, & Maintain)

Image Credit: Moto “Club4AG” Miwa via Flickr
Image Credit: Moto “Club4AG” Miwa via Flickr

Reef Tank Alkalinity (How To Raise, Lower, & Maintain)


In reef aquariums, alkalinity is important to ensure a healthy growing environment for coral. Proper alkalinity levels also have a buffering capacity to help prevent pH swings in reef aquariums. However, as we have covered in the past, everything in saltwater aquariums is connected and the same goes for alkalinity.

Saltwater aquariums are a balancing act as we attempt to match the natural environment of coral reefs in our homes and offices. It requires that we are constantly testing out tanks to ensure a level of alkalinity is within the acceptable range of 8-12 dKH.

Reef Tank Alkalinity Range

Typical home reef aquariums require alkalinity be maintained between 8 and 12 dKH. Natural reefs have a dKH between 8 and 9 but a healthy aquarium reef can drift to as high as 12 dKH without worry.

In order to properly maintain alkalinity in saltwater aquariums, it is important to have a basic understanding of alkalinity is and the relationships it has with other elements in the tank, like calcium and pH, and how those elements can increase and decrease alkalinity based on the ratio of their presence in the tank.

What is alkalinity in a reef tank?

Alkalinity is something that you have probably heard all of your life but you might not know much about it and why it is important for the aquarium hobby. Alkalinity level is essentially the final measurement of alkaline substances in the water and helps to indicate how well the water will be able to neutralize acids.

What are alkaline substances? Substances like carbonate, bicarbonate, and hydroxide are all a part of alkalinity.

What is dKH?

dKH is a unit of measurement for water hardness. dKH stands for degrees of carbonate hardness. It uses a “k” instead of a “c” because it uses the German word for carbonate which starts with a “k” – Karbonathärte. You may also see dKH represented as ºKH as well. This means the same thing and is just using the degree symbol in place of the “d” for degree.

What should the alkalinity level be in a saltwater fish tank?

Ensuring that you have the proper level of alkalinity in your reef aquarium is crucial to ensuring the success of your corals.

Seawater measures 8 to 9 dKH. If natural seawater measures somewhere between 8 and 9, that means you should be doing everything in your power to ensure that your reef tank is matching those standards as closely as possible.

However, the recommended range is a little more forgiving with home reef tanks usually being allowed to be between 8 and 12 dKH but we warn you that 12 is pushing it. Calcium carbonate is also represented in reef tanks as parts per million (ppm). Alkalinity’s recommended range in parts per million would be between 125 and 200 ppm.

How does alkalinity level affect a reef tank?

Alkalinity can affect a reef tank in several ways depending on the levels at which it is present in your reef tank. At high levels, alkalinity risks “burning” your corals. However, it isn’t exactly a “burn” like you may be thinking.

Alkalinity burns happen because, in conjunction with PAR in the tank, the photosynthesis process is increased but the corals can not fully match the growth. Imagine if human bones were able to outgrow our muscle and kin growth. That is what it is like for the corals as they attempt to keep up with the skeletal growth but they end up with tissue loss.

There is also a close link between high alkalinity and a reduction in calcium. Calcium is needed for the successful growth of your corals as their skeletons are largely made up of it.

However, there is some good news because when calcium and alkalinity are just right, stimulates the growth of coral and algae in your tank. This is why it is so important to manage the levels of your alkalinity closely.

How does the alkalinity level affect ph in a reef tank?

Alkalinity can also affect the rate at which pH is increased in your reef tank. High alkalinity can act as a stimulant to increase pH in the tank and thus create further problems for your corals and fish.

Is a high alkalinity level bad for a reef tank?

High alkalinity is a pretty big problem if it persists in your reef tank. There are numerous problems that can stem from alkalinity being too high like alkalinity burns, high pH, and a reduction in calcium levels in the water. If you notice that your alkalinity is too high, you should take steps immediately to help reduce it to acceptable levels again.

Why does the alkalinity level drop in the reef tank?

Alkalinity mainly drops in reef tanks for three reasons. The first reason is that your corals are naturally consuming the alk in the tank which requires replacing periodically. The other reason is that you just added calcium to your tank and the disproportionate addition created a down spike in alkalinity. Finally, maybe you performed a water change that happened to have really low alkalinity (or very high calcium), and that caused the lower levels of alkalinity.

What causes high alkalinity in a reef tank

High alkalinity can be caused in a handful of ways. The first and most likely cause for your high alkalinity level is that the calcium level is way too low in comparison. Since this is the most common reason for the increase in alkalinity then a great way to balance it out is to increase the compensate the calcium level in the tank.

It can also increase due to quickly declining nitrate levels in your reef tank. Test your nitrate levels to see if this is the reason for the increase in alkalinity in your reef tank. if your nitrate level has fallen by 50 ppm or more there could have been an increase in alkalinity by as much as 2.5 dKH.

How to test the alkalinity level in a saltwater aquarium

A successful aquarist is able to keep accurate measurements of all their water parameters, including trace elements in their tank. Before you start acting like you have an alkalinity issue, you need to verify whether your alkalinity is too high or low in your reef tank. Always make sure to have test kits in your aquarium toolbox for emergencies.

There are many test kits available at retailers and they all range in their ease of use. Get a test kit that you believe will be easy to follow and won’t cause you issues in discerning the outcome. There are some tests where the colors can be very close in shade, it can be difficult to determine the correct measurement.

How to raise the alkalinity level in saltwater aquarium

The number one way to raise the alkalinity in a saltwater aquarium is to add baking soda to the water. However, there are also products available commercially that should specialize in increasing the alkalinity within your tank. Alternatively, you can try to buy salt mixes that have a higher dKH and perform small water changes to see if that will help to increase the alkalinity.

Additionally, you can add more substrate in the form of seashells to the bottom of your tank. The seashells will give off more calcium and magnesium which will also act as a buffer for the water. Use about 2 pounds of seashells per 10 gallons of water; however, you might consider adding more depending on how your reef tank reacts.

How much baking soda does it take to increase alkalinity in a reef tank?

It doesn’t take much to stabilize the alkalinity in reef tanks with baking soda. You should use about 1 teaspoon of baking soda per 10 gallons of water in your reef aquarium. Once it has been added, you just wait 24 hours and test the water again to see where the alkalinity stabilized at. Baking soda can be one of your best friends in the hobby. If you don’t cook or clean with it often, you should definitely try to keep some in your aquarium toolbox for situations like increasing your alkalinity.

How to lower alkalinity in saltwater aquarium

Lowering the alkalinity in your reef tank can be a pretty simple process once you know-how. Performing 20 percent water changes can make a significant difference. However, watch what kind of salt mix you are using to make the new water for your reef tank. Salt mixes all come with different levels of dKH so make sure the one you are using is on the lower side. The last thing you want to do is introduce water that has just as high alkaline levels back into the reef tank.

For a more immediate effect, you might consider dosing your tank with white vinegar. This will also affect the pH so if you want to try to keep the pH stable, you will need to disturb the surface of the water to promote gas exchange. you can do this automatically by aiming your powerhead fans toward the surface of the water.

If you don’t want to go through the hassle of performing water changes or adding white vinegar which can negatively affect the pH levels in your tank, you can just wait. The corals in your tank will reduce the alkalinity over time as long as you do nothing. It should reduce as much as 1 dKH per day. However, if you have already been doing nothing and notice that your alkalinity is high, then you might need to take a more proactive approach.

How do you keep alkalinity stable?

The best way to ensure that your alkalinity remains stable is to introduce a few of these practices into your regular care routine. Consistent practices are how to ensure a system is more stable, but there will be times when anomalies happen.

Using a 2 part calcium and alkalinity solution is what most reefers lean towards to ensure their system is stable. These solutions can be found at most reef tank retailers and can make a huge difference in ensuring that your system is consistent. Make sure you are using the minimum recommendations to start out. Between each part, there should be a ten minutes window.

After both doses have been added, wait about 24 hours before you test your system again. From there, you can determine if you need to increase the dose above the minimum recommended dosage.

Does raising calcium lower alkalinity?

Yes, since the calcium and alkalinity are so closely linked in the chemistry of the reef tank, raising your calcium levels directly can lower the alkalinity. Why this happens is because as the calcium in your reef tank increases, so does the precipitation of calcium carbonate.

Of course, the opposite effect can happen as well. This is why there are two dose solutions available with both of these elements in them as it will ensure that both are being raised to a particular point without driving the other one down.

Does alkalinity change throughout the day?

Alkalinity does change slowly throughout the day as your corals take in the alk. However, their consumption rate is not consistent and can also be affected by the time of day. It all depends on your corals and what you are adding to your tank.

The addition of calcium can lead to excess precipitation of calcium carbonate which in turn will lead to lower alkalinity. You may be trying to help your corals by providing additional calcium in the water to promote growth but without compensating for the alkalinity, you are doing more harm than good.

Final thoughts

Monitoring the water parameters in your reef tank is incredibly important not only to promote coral growth but to ensure that coral safety. Alkalinity is important in reef aquariums to help maintain healthy growth without them becoming injured in the process. You always want to make sure you have test kits on hand to ensure that you are staying within acceptable water parameters in your reef tank.

If you are new to the aquarium hobby and have started to get into caring for corals, learning how to properly maintain water parameters like alkalinity will greatly increase the health and stability of your system as a whole. While you may not be able to keep your alkalinity perfect all the time, your corals and fish can survive for a period of time without the ideal conditions. If you notice that your alkalinity is off, don’t fret. You have some time, so ensure that you have identified the problem and begin to tackle it head-on, rather than trying to do whatever is within hands reach to solve the problem.

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