How does Alkalinity affect my reef tank?

Published 2018


Written by Dan

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 this article covers







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.


How do I raise my alkalinity?

The easiest way to increase your alkalinity is by adding carbonates to your tank.  This can be done by adding either sodium bicarbonate or soda ash.  Both will increase your alkalinity by adding in carbonate and bicarbonate minerals however each has its own side effects to be mindful of. When adding sodium bicarbonate to your tank you can expect a temporary drop in PH.  This PH drop is short lived and will move back up after a few hours. Soda ash in comparison will have a near immediate increase in PH.  Do to this PH increase you have to be mindful of how much you add at any one time.  It would be better to add this in small doses over many days.  I would be even better to do this with a dripper or dosing pump.  

Use Kalkwasser to maintain a sonsistent Alkalinity and PH level

Kalkwasser is simply a calcium hydroxide powder that you can mix into freshwater and then add to your tank.  It should be added slowly over time for best results through a dripper, dosing pump, or my favorite to simply mx it into your auto top off reservoir.

Kalkwasser is different from sodium bicarbonate and soda ash because it does not actually contain any carbonate or bicarbonate.  Rather it contains calcium and hydroxides.  The calcium will simply help maintain your calcium level but the hydroxides will react with the carbon dioxide CO2 in your tank to form carbonates and bicarbonates.

When mixing your kalkwasser solution mix somewhere between .5 and 2 tablespoons of kalkwasser per gallon of freshwater.  The water cannot absorb more than 2 tablespoons so any more than that is a waste.




Use a two part dosing method to maintain consistent alkalinity

A two part method simply means you are dosing calcium and alkalinity in two separate mixtures.  The calcium mixture is made by mixing calcium chloride into freshwater.  The alkalinity mix will is generally some combination of sodium carbonate or sodium bicarbonate mixed in fresh water.  The alkalinity mix is the same as we discussed earlier when we talked about raising your alkalinity.  When attempting to maintain your alkalinity rather than significantly raise it you will drip this slowly into the tank over time so that you are adding back in what your coral is taking out each day.

How do I lower my alkalinity?

We spent a good amount of time talking about how to raise your alkalinity since that is the most common situation you will find yourself in.  There is however the chance you could raise your alkalinity to high.  This can happen by accidently dosing to much two part or if a dosing pump or auto top of pump malfunctions and dumps to much kalkwasser solution into the tank. When this happens and your alkalinity is too high, the best method for lowering it is to do a water change.  A batch of fresh saltwater is the best solution and is another good reason to keep some on hand at all times.  


To recap, alkalinity is simply the measure of how much acid is needed to drop your PH from its current point to around 4-5.  The existing carbonate and bicarbonate in your water act as a acid buffer and the more of them that are in the tank the high the alkalinity is or the more acid is needed to drop it that 4-5 range.

Alkalinity can easily be maintained in combination with your calcium level by either using a kalkwasser or two part mixture routine.

Is there anything else people should know about Alkalinity?  Let us know in the comments below.







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