Buffer Capacity and Buffer Collapse.
Buffer capacity refers to the ability of a solution (and aquarium water is a weak solution) to maintain its pH value when an alkali or acid is added. The buffer capacity of aquarium water is not measured; it is only characterized as “weak” or “strong”. There are solutions with a weak buffering capacity, where each added drop of acid or alkali changes the pH, and there are strongly buffered solutions capable of maintaining a constant pH despite significant chemical additions. Of course, under certain conditions, even water with a strong buffering capacity begins to change its pH, but it is important to understand the characteristics of this property and use it correctly where necessary.
Any changes in pH levels should be made smoothly and gradually! Poorly buffered water will be more prone to sudden pH fluctuations than well-buffered water. A buffer can be considered a substrate that contains a lot of limestone to maintain high water hardness. Aquarium gravel often consists of calcium-containing material that increases water hardness and pH. A piece of wood or driftwood can also act as a buffer, acidifying the water and lowering the pH. Note that buffering and alkalinity are closely related, as alkalinity essentially acts as a buffer that slows down pH changes in water. As a rule, hard water usually has an alkaline composition (pH above 7) and serves as a good buffer. At the same time, soft water (pH below 7) is somewhat more acidic and is not suitable as a buffer.
Carbonate hardness is the concentration of bicarbonate and carbonate anions in water. KH and pH are interconnected, and therefore it is logical to consider them together. The relationship between these parameters is very simple. The higher the bicarbonate content in the water, i.e., the KH value, the higher and, most importantly, the more stable the pH value; for example, calcium bicarbonate is an excellent water buffer. Pay attention to the word “stable”. In this context, it means that water with high carbonate hardness, over 12 degrees, has an alkaline reaction. Water of medium hardness (6-12 degrees) will likely be close to neutral (pH ~7.0), while water with a carbonate hardness of less than 6 degrees KH can basically be anything, but most likely will be acidic.
Buffer collapse is a sharp change in pH, most likely in an alkaline or near-alkaline environment when the pH approaches 7. In this case, due to the alkaline reaction, harmless ammonium turns into ammonia, and fish, especially young ones, quickly die. This phenomenon is usually observed in overstocked aquariums when fresh water is added. In such aquariums, changes in pH are influenced by oxidation processes caused both by protein compounds, which are present in excess in overstocked tanks, and by accumulating metabolic products, both visible and invisible (soluble). Water should be added to such aquariums in small portions, and the pH of the added water should be below 7. In water that is too hard and alkaline (pH above 9), everything is also bad.
First, as water alkalinity increases, a greater amount of ionized ammonium turns into free ammonia dissolved in water. Ammonia is many times more toxic than ammonium, so seemingly minor increases in ammonium concentration can unexpectedly cause severe poisoning of the entire aquarium. Preventing this is simple—change the water more often, or use special water conditioners. Second, as the alkalinity of the environment increases, fish experience impaired oxygen uptake. In an alkaline environment, fish can suffocate even when dissolved oxygen levels seem quite sufficient. And, of course, alkaline water itself (especially with a sharp increase in pH) causes burns and damage to the external organs of fish.
In freshwater aquariums, the buffering level depends on the level of carbonates and bicarbonates in the water. Thus, the terms carbonate hardness and buffer capacity of water are used interchangeably. Although theoretically these are not the same thing, they are practically equivalent in terms of fish keeping. Buffer capacity refers to the ability of water to keep the pH level stable when both acid and alkali are added to the water. If the water has a sufficient buffering level, the buffer capacity can absorb and neutralize the added acid without a significant change in pH. Figuratively speaking, the buffer acts like a large sponge. When acid is added, the sponge absorbs it and prevents the pH level from changing. The sponge’s capacity is not limitless, and once the buffer capacity is exhausted, the pH begins to change rapidly with further additions of acid.
Buffer capacity has both positive and negative qualities. On the positive side, when the nitrogen cycle produces nitrate, it acidifies the water. Without buffering, the aquarium pH would drop after a while (which is bad). With sufficient buffering, the pH level remains stable (which is good). On the other hand, water (especially hard water) has a high buffer capacity. And if the water pH is too high for your fish, the water buffer capacity prevents the pH from lowering when you try to do so. If you ignore the buffer capacity of the water, attempts to change the pH in the aquarium are doomed to failure.
What level of buffering is necessary for an aquarium? In most cases, test kits designed to measure buffering actually measure the water’s KH. The higher the KH, the more resistant the water will be to pH changes. The KH in the aquarium should be high enough to prevent large pH fluctuations over time. If the water KH is below 4.5° dH, you must constantly monitor the pH level in the aquarium to ensure it does not change within dangerous limits. This is especially important if you do not perform regular water changes. In particular, the nitrogen cycle tends to decrease the pH level over time.
How much the pH will drop depends on the amount of nitrates produced, as well as on water hardness. If the pH drops by more than 0.2 units per month, it is necessary to increase water hardness or perform more frequent water changes. In addition, an important feature of water should be taken into account – its ability to self-heal. This is especially true for hard water. For example, by diluting tap water with pH = 7.5 by 50% with distilled water and thus lowering the pH to 6.5 or even lower, you can be sure that after a few hours the pH will return close to the original 7.5. Water KH does not directly affect fish, and usually, there is no need to precisely maintain a fixed hardness level for specific fish.
In an aquarium with Rift Valley cichlids, it is necessary to provide sufficient water buffering with calcium and magnesium salts, as accidental overfeeding can easily lead to the saddest consequences. River African species easily tolerate water acidification below 7.0, but still feel better in alkaline water. It is necessary to maintain ideal cleanliness and a stable chemical composition of the water.
The effect of pH on the life of fish and plants.
pH – KH – CO2 equilibrium. Determination of the active reaction of water. 
