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December 5, 2023
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News & Reviews Article
Title pH Alkalinity and Calcium in Reef Tanks

Ralph Cabage

Last Updated



pH Alkalinity and Calcium in Reef Tanks

pH Alkalinity and Calcium in Reef Tanks


Explaining pH is complex and varies depending on the context in which it is discussed. Simply put, it's a measure of the acidity or bascicity of a solution. Solutions with a pH <7 are called "acidic" and those with a pH >7 are called "basic." The pH of natural sea water varies throughout different marine environments. Reef tank pH should be maintained between 8.2 and 8.3. pH in a reef tank will fluctuate throughout the day. It will rise during the day and fall at night. pH test kits come in many different brands and types. Most of the brands work fine but some are easier to read than others. "pH pens" or digital pH monitors are quite helpful, especially to new aquarists. Considering many pH test kits sold for the hobby cost anywhere from $7 to $20 dollars for ~20-30 tests, investment in a pH pen is economical.

Low pH is most often caused by poor aeration, and/or low alkalinity levels. Poor aeration can be remedied with more water flow and turbulence at the surface of the water. Protein skimmers and overflows also help with aeration. Dosing of limewater (kalkwasser solution) often helps.


Reef tanks need calcium (not just for stony corals but for many soft corals and other reef critters too). Natural sea water has a calcium concentration of 350 to 420 ppm. Calcium levels differ by sea and area of the world and calcium needs will depend on what type of corals you are keeping. If you are keeping fast growing stony corals (such as Acropora, Montipora, etc.) you should strive to keep your calcium levels at 380 to 420 ppm. However, if you are keeping slow growing stony corals (such as Fungiidae, Faviid, etc.) or soft corals, calcium levels as low as 350 ppm are usually acceptable. Reef tank calcium should be maintained to at least 350 ppm (and at 410 to 420 ppm for tanks with faster growing stony corals). Calcium levels can be too high. High calcium levels bring down alkalinity. It's difficult to have a calcium concentration of 450 ppm or higher while also maintaining sufficiently high alkalinity. Imagine that calcium and alkalinity are on opposite ends of a scale. When one goes up, the other goes down. You can raise your calcium with CaCl (Calcium Chloride), Ca(OH)2 (aka kalkwasser), or with a calcium reactor. Please see below for information on alkalinity.

Buffers and Alkalinity:

Buffers help protect your aquarium water against sudden changes in pH. The ability of the buffer to prevent sudden changes in pH is called a "buffering capacity." As the term is usually used in the hobby, alkalinity can be thought of as one measure of the buffering capacity of our aquarium water. Chemical compounds called "bicarbonate" (i.e. baking soda) and "carbonate" are the major contributors to alkalinity. They're not the only contributors, but in our tanks, they're by far the most important. They're also what is usually tested for in alkalinity test kits sold in the aquarium hobby. Reef tank alkalinity should be maintained between 2.5 and 4 meq/L (or 7 and 11 dKH). It's important to note that while alkalinity helps your tank resist changes in pH, buffers can be "broken" if pH rises or falls too much too quickly. Therefore it remains important to monitor pH so that it does not fall too far out of the acceptable range. The most common way to raise alkalinity is with commercially sold reef aquarium buffer product.

Brief guide to adjusting calcium and alkalinity

With calcium <300ppm and alkalinity 0 to 6 meq/L (0 to 17 dKH) :
This imbalance is usually caused by over-dosing buffers. To correct this problem, add calcium chloride slowly. It's very important to test the water before and after every dosing. Do NOT use kalk to correct this problem. Good products include Calcification, Precision Calcium or Calcion.

With calcium 300-400ppm and alkalinity 1 to 2 meq/L (3 to 6 dKH):
In this situation both your calcium and alkalinity are on the lower side. Dosing kalk solution and/or some brand of 2-part calcium/buffer solution should solve the problem. Good products include Reef Fusion, C- Balance, or Reef Code

With calcium >450ppm and alkalinity >4 meq/L (>11 dKH):
When both calcium and alkalinity are too high, all you have to do is refrain from adding any calcium or buffer supplements and wait for the problem to correct itself.

With calcium >450ppm and alkalinity <2.5meq/L (<7 dKH):
When calcium levels are too high and alkalinity too low, dose Aquavitro 8.4 or Reef Builder (if pH is normal). If pH is low is still low use AquaVitro Balance or pH+ by Brightwell to correct the problem.


As you will come to experience, pH, calcium and alkalinity interact with each other. As mentioned previously, high alkalinity results in lower calcium levels and vice versa.Because of this, calcium and alkalinity must be measured at the same time and adjusted with respect to each other. Too much or too little of one is often a sign that the other is too low or too high respectively. Low pH can be, though is not always, a sign of low alkalinity. It can also be a sign that the water is not aerated adequately. When your tank's water chemistry is not right in some respect, you may have to do some problem solving to figure out 1) what's causing the problem and 2) how to correct it.

Products related to this article
Reef Buffer 250g
Reef Builder 300g
Kalkwasser 500g 1lb Jar

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