pH (always properly written with little p, capital H) is the concentration of H+ ions. If you don't know chemistry, what that really means is how acidic or how basic (a.k.a. alkaline) the water is. A pH of 7.0 is considered neutral. pH>7 is called basic and pH<7 is acidic. How this concerns fish is that fish come from different waters across the world. The Amazonian basin is mostly very acidic, with pHs around 5 and even sometimes as low as 4. By contrast, the Rift Lakes in Africa is very basic, pHs over 8 and some parts as basic as 10.
The experienced fishkeepers will always research a fish's natural habitat before purchasing a fish, so they can prepare a tank that replicates as closely as possible the natural water. As you can imagine, a fish is at its best when living in water that is the same as its natural water.
Now, given that a fish will do best in water that matches its native water, many, many fish can adapt and live long and healthy lives in a pH that is not exactly the same as its natural water. This is where research is vitally important. As an example, the common goldfish is very tolerant of a wide range of waters, whereas many discus can only be kept under very specific conditions.
What is of vital importance is not to match most fish's natural pH, but to give them water what the pH stays constant. This is where KH (properly written with both letter capital) comes in. KH is a measure of a water's buffering capacity, where buffering means ability to keep a constant pH. Some synonyms for buffering you may also see on the web are alkalinity and acidity. Note that there is a difference between the words acidic and acidity -- acidic means pH<7, acidity is the water's capability to keep a constant pH. Same thing with alkaline and alkalinity.
So, since it is important to keep a constant pH, what you want is a high KH, so that you know that the water will keep a constant pH for a longer time. See, the natural processes in an aquarium are trying to make the water more acidic all the time. The end process of the cycle are acids, and adding more of those will lower the pH. So, over time, the buffering capability will be consumed keeping the water at a constant pH and when eventually all the KH is used up, the pH of the water starts going down. This is also known as pH crash and can happen pretty quickly if you are unaware. And, back to the previous paragraphs, that changing pH is very stressful for the fish.
From all that, I hope that you have learned that pH and KH are very separate statistics the describe the condition of your water, though they are related. In general, hard, basic water has a high KH, and soft, acidic water has a low KH, though that is just a generality, it is not true in every case. The previous poster said "controlled" which I don't think is quite right. Controlled implies that if you change KH, you can change the pH. Which isn't true, you can definitely increase and decrease your KH without changing the pH. That is why I like the word related instead of controlled.
Keeping the KH high is pretty easy. Most tap water has a pretty decent KH value, and when you do your weekly water change, the new water will replenish the KH of the tank. If you have a very low KH, you may have to do more frequent/larger water changes, or perhaps explore some chemical additives for your tank to increase KH. In general, most of us do not like adding chemicals to a tank unless as a last resort, so I'd try the water change route, first.
Finally, the real question here is 'what is too low?' well, it is good that you have a test kit there, since having real numbers adds some definitiveness to this question. Take a sample of your tap water and sit it out for 24 hours, then test that. Sometimes water companies add CO2 which lower the pH & KH of the water, so a test immediately out of the tap is not a good indicator the the water that will be in your aquarium day in and day out. Part of the problem with just trusting the LFS in their saying "low" is that they do want to sell you extra chemicals and make a sale. I'm not saying they are wrong, I just personally would really like to know the number itself rather than the employee's interpretation of the number.
So, post the results of the test, and the forum can tell you if the water's KH is really low, and maybe give you some strategies for keeping it high pretty easily.
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