How does Alkalinity Affect my reef tank?


One of the biggest impacts on whether your reef tank thrives or is just kinda ugh is most definitely keeping your Alkalinity in check. Honestly though it is one of the most confusing things for new hobbyists. Let’s try to simplify it!

What is the definition of Alkalinity?

Alkalinity is the measure of how much acid can be added to a solution before it drops to a PH of about 4.2. Therefore it is a measure of anything in the tank that will buffer acid.

Why is Alkalinity important to my reef tank?

Truly it isn’t alkalinity that is important to us. Testing alkalinity though, is the best simple way to test what is important. Carbonate and bicarbonate are what is truly important to the reef keeper. Testing for alkalinity gives us a good indication of the carbonate and bicarbonate levels in the tank.

The other aspect of alkalinity is that by having a proper range of alkalinity you also are able to more accurately maintain a desired PH. If alkalinity falls so will the PH and vice versa.
Carbonate and bicarbonate are used by coral, invertebrates, and even coralline algae to grow. They will use these in combination with calcium from the water to build up their skeletal structure.

If you have ever seen a dead stony coral you know it is made up of hard calcified white stone like substance. This is what they are making with the carbonate, bicarbonate, and calcium. It is similar to how your body makes your bones out of calcium.

Without these materials your coral will eventually wither away and die. This is the ultimate reason alkalinity is important to your reef tank.

How does an alkalinity test work?

Most hobby grade tests will include a bottle of carbonic acid mixed with dye that changes color based on the PH of the water it is in. To perform the test you add drops of the test solution one by one into your sample water from your tank. As you do the water will change colors. Most kits on the market will go from a blue or purple to a yellow when it is done.
You simply count how many drops you had to add to get to yellow and then look at a chart like the one below from API to find out your alkalinity.

How does testing Alkalinity tell me how much carbonate and bicarbonate are in my tank?

There is no simple way to test for carbonate and bicarbonate directly without some fairly expensive equipment or very extensive processes. These just are not feasible for the aquarium hobby at this time. The next best thing is to test for alkalinity.

As stated above, to test alkalinity you add a test solution containing a carbonic acid to your water sampling. The acid will mix with your water sampling creating different chemical reactions which will once done drop the PH.

Typically most of the chemical reactions that take place are caused by carbonate combining with carbonate acid to create bicarbonate, and bicarbonate mixing with the carbonic acid to create more carbonic acid.
The problem there is that I said most of the reactions and not all. They do however make up more than 95% of the reactions on average. This means that total alkalinity (TA) is not a true measure of carbonite and bicarbonate in the water. It does however get us close enough. I know the old saying about close only being good in horse shoes and hand grenades but I’m adding alkalinity to the list.
Since our measure of alkalinity is only a close approximation to what we are really concerned with, most hobbyists will aim to be in the middle of the given range of their test kit so they have more wiggle room.


A well-written Article by Dan

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