Re post #3
KH is not potassium. K, the chemical symbol for potassium, is from Kalium, a Latin word.
The K in KH comes from Karbonate (a German word), in English: Carbonate. It is a couple of different forms of minerals with a carbon and 3 oxygens stuck on. The CO3 is the carbonate part.
CaCO3 is Calcium Carbonate, a very common material on Earth.
It also relates to bicarbonate, HCO3. This is found in many living systems. You might be familiar with bicarbonate of soda, or sodium bicarbonate, which is baking soda.
The link in the aquarium between carbonate, bicarbonate and carbon dioxide is a bit complex, but here are a few simplified concepts:
Use a separate test for pH, GH and KH. Neither one measures anything the others measure. While there may be links, and related results, the rule is NOT ALWAYS. You can find charts that relate the level of KH and the pH to the level of CO2. These charts are not always accurate, because there are other buffers in the water besides carbonates.
Fish are usually referred to as Soft Water or Hard Water. This is GH. Fewer minerals in the water = soft water. The primary minerals measured are calcium and magnesium. Fish and plants use these minerals, so if the level is really 0, this is a problem. If you need to alter the water to suit the fish it is best to set the GH to the right level first.
KH is a buffer that stabilizes the pH. With high KH the pH tends to be high. With low KH the pH can vary more, depending on what else is in the water. There are other buffers in aquariums, too, but KH is one of the most common. Once you have the right GH in the tank, make the KH pretty much similar, within a couple of degrees.
pH is a ratio of hydrogen to hydroxyl ions, that is H+ to HO- in the water. When there are more H+ the water is called acidic, and when there are more OH- the water is called alkaline. One example of why pH can be important: Ammonia is most common in the aquarium in either of 2 forms. When the water is more acidic, that is low pH, the ammonia is present in the form called Ammonium, NH4, and is less toxic to the fish. They can keep it out of their system easier. When the pH is high, that is alkaline, ammonia is present in the ammonia form, NH3, and is more toxic. It enters the fish through the gills, and is toxic.
Here is the link between GH, KH and pH in natural water systems (and it is not a hard-and-fast rule)
As rain falls over the rocks and seeps through the soil it can dissolve the minerals in that soil and the rocks.
In many places on Earth there are rocks that are high in calcium carbonates and magnesium carbonates. There are different forms of these, some more soluble than others.
If the rain is plentiful it may have already dissolved most of the soluble minerals ages ago, so what is left is not very soluble. With plentiful rain there is almost always lots of plant growth, so the rain may not fall on actual soil at all, but falls on a deep bed of fallen leaves. This water is soft. It has very few minerals. Low GH and KH. The pH is usually low (acidic) because of the decomposing leaves.
If the rain is sparse, then when it does fall, it may land on rock that has not been dissolved over time, so more of that rock dissolves with each rainfall. Also in areas with sparse rain there are not so many plants, so the rain is more likely to fall on the rocks and soil. If these rocks and soil are high in the various carbonates such as calcium and magnesium carbonates, then the run off water becomes rich in these minerals. It has high GH and KH, and almost always high pH.
Now, the importance to aquarium keeping:
Fish live in balance with their environment.
When there are not many minerals (of any sort) in the water the water keeps entering the fish cells through the gills and digestive tract. Too much water enters, so the fish needs to get rid of it. Fish from soft water are very good at getting rid of the excess water, and at retaining the minerals they need from that water, even when the minerals are there in very small amounts. These are soft water fish. Mostly they thrive in water with low GH and KH, usually acidic water. (The pH is less important). Some species are so good at trapping minerals they can die when they are in hard water, the minerals build up in their system faster than they can use them.
Fish from hard water also have some water entering their cells, but a lot less, so their bodies are not very efficient at getting rid of excess water. If they are placed in water that is too soft they can swell with a condition called Dropsy. (There are other things that can lead to dropsy, but the inability to get rid of excess water is the basic problem). These fish thrive in water with high GH and high KH, and almost always water with high KH also has high pH.
However: GH, KH and pH are separate things, though there may be some trends where one test result is similar to the other. Measure them with separate kits. They each mean something different in the aquarium.