Depending on how much they know, aquarists tend to change the chemical make-up of aquarium water more or less. Even though these parameters can and often must be changed as needed, there are some compounds, like pH buffers, that must be in an aquarium. Getting rid of them all would be a bad idea.


What bicarbonates do in an aquarium

Bicarbonates are important in any body of water because they do two important things. Let’s start with a theory to explain both of these things. Carbonic acid is the acid that makes carbonic acid. We all come across carbonic acid (H2CO3) every day. It is made when carbon dioxide (CO2) and water (H2O) come together (H2O ).

CO2 + H2O ↔ H2CO3

The compound carbonic acid is not very stable. It breaks down very quickly when things like pressure or temperature change (bubble effect after unscrewing a bottle of sparkling water). Acids always break down (dissociate) in a slow, steady way. In this case, it happens in water, so there is always a water molecule in the equation.

H2CO3 and H2O make H3O+ and HCO3–, which is a bicarbonate ion.

The carbonate ion is made when HCO3– + H2O = H3O+ + CO32-.

In each of the above equations, H3O+ is present. It is a cation called oxonium, which is made up of H+ and H2O. As a way to start, it’s important to know that the pH scale used to go from 0 to 14, but now it’s easy to find things like compounds with pH = -2.

The acid or base reaction is caused by the H+ and OH– ions. The air is more acidic when there are more H+ ions in it. On the other hand, the pH goes up when there is more OH–. (alkaline environment). Because of this, it is easy to see that the amounts of H+ and OH– are closely related, and that when their amounts are equal, pH=7 (neutral).

Aquarists call bicarbonate transient hardness (KH), and this is the only way it is thought of. There will be more of them in one aquarium and less of them in another. Because they are always in the water and because of how they are made, they are a great source of carbon dioxide (CO2) for plants. I wrote more about this in the article Calcium deposits in the aquarium and biogenic decalcification. So, today we will only talk about the most important thing they do for water, which is to keep the pH stable. Because bicarbotanets are the aquarium’s most basic pH buffers.

How do buffers keep the pH of the water in an aquarium stable?

Most people think of using acids (which add H+ ions) or alkalis (which take them away) to fix acidity (increasing OH–).

Remember that pH is not the same as temperature. Life in the aquarium doesn’t change much when the water temperature goes up or down by 1 degree. But in the definition of pH, only the decimal logarithm is used. This means that a change of 1 in pH, such as from pH = 6.5 to pH = 7.5, is by definition a change of a whole order of magnitude.

Let’s say we’re standing in the middle of a meadow. The temperature is 200C and the sun is shining. Perfect, neutral, like a pH = 7 aquarium. The weather changes all of a sudden. The temperature drops ten times, to 20C. We didn’t have enough time to get ready. We’re still standing in a T-shirt and asking, “Where did the perfect weather go?” Well, the pH of the water in the tank just dropped to 6. In practice, a difference of 1 point is a big deal. What do we get out of going to the aquarium? The fish swim close to the surface, have gills that are very red, and have trouble breathing.

The careless and inexperienced aquarist decides to save the situation by adding a product that raises the pH, but the manufacturer didn’t say how much to use on the package. It makes sense, since the manufacturer can’t know how much an aquarist will want to change the parameter. This should be done carefully, and a test should be used to keep a close eye on the situation.

What happens if we do too much? Let’s say that the pH goes from 6.0 to 8.0. Right now, the temperature in the meadow went from 20C to 400C. There’s no doubt that sudden changes in temperature like this will cause a drop in immunity and the flu or a cold in people. Fish live in water, and in this situation, careless actions will cause a lot of bad things to happen.

This kind of thing can also happen in lakes and rivers. For example, decaying organic matter brings the pH down, while a lot of photosynthesis brings it up. So, nature has come up with ways to prevent these changes to some extent. Bicarbonates are there to help.

Bicarbonates have a buffering effect.

When strong acids or bases are around, they react with them to make them neutral. Importantly, you only need one compound for an acid and a base to react with. The plan for this kind of reaction is always the same and looks like this:

Bicarbonate plus an acid or base makes an inert molecule (precipitate or gas) plus salt and water.

Ca(HCO3)2 + 2HCl = 2CO2 + CaCl2 + 2H2O. The CO2 molecule “escapes” into the air or is used by plants in the aquarium.

Ca(HCO3)2 + NaOH → CaCO3↓ + Na2CO3 + 2 H2O (CaCO3 is a precipitate, very hardly soluble in water)

Pay special attention to the fact that, even though strong acids and bases are involved in the reaction, no H+ and OH– ions are made. These are the ions that control the reaction (pH) of water. These ions, which come from HCl (a strong acid) and NaOH (a strong base) in the above reactions, are neutralized right away by the bicarbonate. This is a one-of-a-kind situation, and pH buffers have never saved a beginner aquarist’s aquarium.

When the water lacks bicarbonates…

Every home tank has what is called a “buffer capacity”. It means only that bicarbonate dissolved in water exists there in a certain amount, and therefore is able to neutralize a certain number of acid molecules. Once all the bicarbonates are used up, there is nothing to stop the acids or bases from drastically affecting the environment.

For this reason, lowering the pH of the tank must also be considered in two ways, as it is this process that causes the most problems among aquarists. We pour many drops into the tank, do several or a dozen tests, and the acidity does not drop. How many people then decided to pour the preparation “by eye” wanting to make their lives easier? That’s how this story came about.

What phosphoric acid does in an aquarium

Lastly, a small side question: How does it happen that, after some time, algae starts to grow in an aquarium where the pH is kept low with acids? How did this effect happen?

Aquaristics is moving away from using hydrochloric acid, the simplest acid to fix acidity, as much as it used to (HCl). Many companies switched to phosphoric acid(V), which is also called orthophosphoric acid and has the formula H3PO4. It is a strong acid that oxidizes things, but because it also has phosphorus in it, aquarium plants can handle its effects. After all, some of us are putting phosphorus-containing fertilizers in our tanks. Let’s look at this example by replacing the above acid with the one shown in the equation:

3Ca(HCO3)2 + 2H3PO4 → 6CO2↑ + Ca3(PO4)2↓ + 6H2O


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