Additives and Alkalinity


Supplementing reef tanks with additives is a hotly debated topic with extrememly knowledgeable people on either side of the issue. I tend to limit myself to adding chemical compounds that will ultimately contribute to high alkalinity and stable pH. The exceptions to that rule are iodine and strontium which I add weekly.

Currently, I dose Kalkwasser as my top off water, and add magnesium ion, and use Phosban to filter out phosphates in order to maintain my alkalinity. Beyond that, I add iodine ion for my clams and strontium for my coral.

Alkalinity *

Alkalinity is the buffering capacity of the water, and is directly linked both the calcium ion content of the water as well as the pH. Alkalinity is directly proportional to the amount of free carbonate (CO3-) and bicarbonate (HCO3-) ions in the water. Said another way, alkalinity refers to the hardness of the water.

Why do I want high alkalinity in my tank?

There are many answers to this question, and I will outline a few.

1. As stated before, alkalinity refers to the acid buffering capacity of the water. In laymans terms, buffering capacity can be thought of as a resistence to pH change, in this case low pH. If the pH goes too low (becomes acidic), then the buffering ions, carbonate and bicarbonate, are consumed in order to prevent a pH swing.

2. Corals require both calcium and alkalinity in order to build their skeletons. Calcium and alkalinity contributing ions are delivered to the reef tank via two part additives like B-Ionic, calcium reactors, or a combination of both. By driving up the alkalinity, a greater amount of calcium ions can be safely accomodated in the water without the risk of the water becoming too acidic. A low amount of alkalinity indicates that the system is consuming too much calcium carbonate, and growth of the stony corals and clams will slow.

By driving up the alkalinity, a greater amount of calcium ions can be safely accomodated in the water...

3. Higher alkalinity will improve the color of the SPS coral.

Can I have too much alkalinity in my tank?

Yes. However, should carbonate become supersaturated beyond the point where the ions can stay free in the water, the carbonate ions will combine with calcium ions and precipitate, or become solid limestone. This is not a terribly dangerous consequence, but the tank parts will become coated in calcium carbonate. This is commonly observed by the owners of washing machines, hot water heater elements, or dishwashers, where the parts become covered in limescale.

Can I have too little alkalinity in my tank?

Yes. This is the more dangerous scenario. Too little alkalinity translates into less resistance to pH swings. If the pH of the water drifts too low, then the health of the tank is at risk. As pH lowers, aragonite (solid calcium carbonate) begins to dissolve in order to compensate for the water's acidity.

How do I maintain my alkalinity?

The easiest way to maintain alkalinity is to use a calcium reactor. A calcium reactor bubbles carbone dioxide gas through calcium carbonate effectively creating carbonic acid (soda water). This carbonic acid dissolves the carbone reactor media (aragonite), and forms the calcium and carbonate ions that we desire.

How do I boost my alkalinity?

Another way to increase alkalinity is with a kalkwasser reactor. Also called a Nielsen reactor, the 'kalk reactor keeps a saturated calcium hydroxide ( CaOH ) mixture in suspension so that the useful portion of the mixture can form at the top of the reaction vessel. Used in conjunction with an automatic top off mechanism, a 'kalk reactor can provide a safe and reliable method of pushing alkalinity levels beyond conventional levels.

What other factors could be inhibiting my alkalinity?

This is not so much a question of adding more carbonate, but more a question of, "How can I use my calcium and carbonate ions more efficiently?"

1. Magnesium - Proper magnesium ion concentration will prevent the precipitation of calcium carbonate. Magnesium ions will "block" other cations like calcium from attaching to carbonate and bicarbonate which would cause precipitation, and thus render the calcium and carbonate ions useless.

2. Phosphates - High phosphates will contribute to the consumption of carbonate through phosphoric acid. Recall that carbonate is a base, so any acidity that is introduced will be consumed by the carbonate. On a side note, phophates are contributors to hair algae which is just another reason to keep the concentration of this ion as low as possible.

Reference material

Randy Holmes-Farley has written extensively regarding reef chemistry. Here are a few of his articles.

Chemistry and the Aquarium - What is Alkalinity?

Chemistry and the Aquarium - The Chemical and Biochemical Mechanisms of Calcification


* In this context alkalinity is actually the water hardness, and should not be confused with the term for a solution with a pH higher than 7.