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This article is from the Beginning Fishkeeping FAQ, by Thomas Narten with numerous contributions by others.

34 Buffering Capacity (KH, Alkalinity)

Buffering capacity refers to water's ability to keep the pH stable as
acids or bases are added. pH and buffering capacity are intertwined
with one another; although one might think that adding equal volumes
of an acid and neutral water would result in a pH halfway in between,
this rarely happens in practice. If the water has sufficient buffering
capacity, the buffering capacity can absorb and neutralize the added
acid without significantly changing the pH. Conceptually, a buffer
acts somewhat like a large sponge. As more acid is added, the
``sponge'' absorbs the acid without changing the pH much. The
``sponge's'' capacity is limited however; once the buffering capacity
is used up, the pH changes more rapidly as acids are added.

Buffering has both positive and negative consequences. On the plus
side, the nitrogen cycle produces nitric acid (nitrate). Without
buffering, your tank's pH would drop over time (a bad thing). With
sufficient buffering, the pH stays stable (a good thing). On the
negative side, hard tap water often almost always has a large
buffering capacity. If the pH of the water is too high for your fish,
the buffering capacity makes it difficult to lower the pH to a more
appropriate value. Naive attempts to change the pH of water usually
fail because buffering effects are ignored.

In freshwater aquariums, most of water's buffering capacity is due to
carbonates and bicarbonates. Thus, the terms ``carbonate hardness''
(KH), ``alkalinity'' and ``buffering capacity'' are used
interchangeably. Although technically not the same things, they are
equivalent in practice in the context of fishkeeping. Note: the term
``alkalinity'' should not be confused with the term ``alkaline''.
Alkalinity refers to buffering, while alkaline refers to a solution
that is a base (i.e., pH > 7).

How much buffering does your tank need? Most aquarium buffering
capacity test kits actually measure KH. The larger the KH, the more
resistant to pH changes your water will be. A tank's KH should be high
enough to prevent large pH swings in your tank over time. If your KH
is below roughly 4.5 dH, you should pay special attention to your
tank's pH (e.g, test weekly, until you get a feel for how stable the
pH is). This is ESPECIALLY important if you neglect to do frequent
partial water changes. In particular, the nitrogen cycle creates a
tendency for an established tank's pH to decrease over time. The exact
amount of pH change depends on the quantity and rate of nitrates
produced, as well as the KH. If your pH drops more than roughly two
tenths of a point over a month, you should consider increasing the KH
or performing partial water changes more frequently. KH doesn't affect
fish directly, so there is no need to match fish species to a
particular KH.

Note: it is not a good idea to use distilled water in your tank. By
definition, distilled water has essentially no KH. That means that
adding even a little bit of acid will change the pH significantly
(stressing fish). Because of its instability, distilled (or any
essentially pure water) is never used directly. Tap water or other
salts must first be added to it in order to increase its GH and KH.

 

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• Index
• next: 35 General Hardness (GH)

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