There is more to pH than a simple test. Water chemistry is somewhat complex. Here is an iimportant consideration in what can hold pH stable or result it it dropping or rising. The info below comes from the FINS site. To learn more about basic water chemistry nin simples terms I usually usggest people have a read here:
https://fins.actwin.com/mirror/begin-chem.html
The general FINS site information is here
https://fins.actwin.com/aquariafaq.html
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 inpractice 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'taffect fish directly, so there is no need to match fish species to aparticular 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.
One other thing to know here is that what constitutes KH in tanks are carbonates and bicarbonates. The bacteria which handle ammonia and nitrite require inorganic carbon to function. The most obvious source for this is CO2, But, when this is not available in sufficient amounts, the bacteria can use carbonates and bicarbonates. When we fail to do water changes it is possible for the KH components to be used up and if this use is great enough, the result is to lower the KH. This is what happens in Old Tank Syndrome. The result is that the pH in such tanks will drop.
For years I have had to set up a number of temporary tanks either for summer use or because I was using them as a vendor at weekend fish events. I do not cycles the tanks individually. Instead, I set up a bio-farm in which I can cycle the filters for 6-8 tanks quickly (about 2 weeks). One of the things I need to monitor and handle is KH. Because there are filters for all the tanks being ccyled in a single tank, it is very easy for the carboneates and bicarbonates (KH) to get depleted. So, I have to take steps to insure this does not happen. I can do this by adding a big bag or crushed coral to the bio-farm.
Coral is basically calcium carbonate. The crushed coral in the farm is dissolved by acid in the water. This then helps to keep the KH remain stable. As the pH becomes less acid, less coral is dissolved. When one is having problems with pH tending to drop in a tank, the solution is usually to add a bag of crushed coral to a filter. Over time as the coral is dissolved it needs to be replenished.
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