This is a response to a request for someone to write an article about basic water chemistry. While there is an excellent article by happychem that can be found here, I'm going to try to simplify things a little. Since I doubt anyone knows me here, I've been keeping fish for about 4 yrs. I currently have a 55g planted community, a 29g planted invert tank, a 10 g convict house and a 5 g hospital tank. During that time period I've kept all kinds of different freshwater fish. Some I've been highly successful with and some have been disastrous failures. I've kept fish very easily that were supposed to be hard to keep and I've killed fish that are supposed to be impossible to kill. Why should I care about water chemistry? Simple, because water chemistry and quality is the major deciding factor in the health of your fish. Basically, water is to fish what air is to humans. If we had to walk around all day in heavily polluted air we'd be sick all the time and fish are the same way. There are chemicals that are more toxic to us than other and fish are the same way. Do I need to be a chemist to keep fish? In short, no. You don't have to hold a chemistry degree, you don't even need to have taken a chemistry class in school (though it won't hurt you.) As a beginner fishkeeper you basically need to be familiar with and be able to measure Ph, Ammonia (NH3), Nitrite (NO2) and Nitrate(NO3). The other terms thrown around less often, but still valuable to know are KH and GH. Ammonia, Nitrite and Nitrate all relate to cycling and are discussed in detail quite capably here. Ok, so what do I need to know about Ph? First of all, Ph is a highly overrated water quality. Ph basically measures the alkalinity of a substance. In scientific terms it lets us know if something is an acid or a base. In laymens terms it measures how acidic something is. The higher the score the less acidic. For example, the acid in your stomach has a ph of 2.0. If you poured it over your hands you would definitely regret it. Bleach on the other hand has a ph of 12.5. You can pour it over your hands without worrying about it eating a hole through your skin. Pure water has a ph of 7.0 which is considered neutral. Water with a ph higher than 7.0 is said to be alkaline. Water with a ph lower than 7.0 is said to be acidic. Another thing to keep in mind is that ammonia is more toxic at a higher ph. Right, but how does ph affect my fish tank? Basically as long as it's stable it doesn't. Fluctuating Ph can be a sign of a number of problems in a fish tank. A Ph that jumps around and is not stable can stress fish, but more likely there are other things going on in the tank that can stress the fish as well. Everyone's water is different. Some people have soft water with a Ph around 6.5. Others have hard water with a Ph of 8.5 or harder. For breeders, it can make a difference, but I'm assuming you're a beginner and just want a pretty tank that people will ooh and ahh over. In that case Ph is not important as long as it's stable and not extreme (less than 6.5 or more than 8.5). Much more important is your ammonia, nitrite and nitrate levels. (You did read the cycling article right?). It is very important to be able to maintain a stable Ph. Okay, how do I do that? In short, that's where Kh comes in. Kh is the measure of bicarbonate and carbonate dissolved in the water. (Yeah, I know that probably meant nothing to you, I was showing off for my science teacher who claims I slept during class.) The term stands for Carbonate Hardness if anyone wants to do more research on their own. I won't get into exactly what it is, but instead, what it does. Kh provides a buffering capacity for the water. Think of a rubber band. You can bend it and twist it and pull it, but as long as you don't snap it as soon as you lay it down it goes back to it's original shape. This is what water with a high Kh does. Think of a paperclip. You can bend it into all sorts of positions pretty easily, but as soon as you lay it down it won't just revert back to it's normal shape. This is how water with a low Kh behaves. Either of these can be a good thing or a bad thing in an aquarium. When I first started keeping fish I had a tank at my parents house. They had well water. The ph of the water was about 8.5 and it was very hard because it came from a limestone well. (Perfect African cichlid water I later realized.) I poured all kinds of powders and liquids into that stupid fish tank trying to get the water to 7.0 and never could. The reason was because the Kh was so high. Ultimately, I left the water as it was and the fish were fine. I kept danios, platies, cories and even an angelfish in that water. The platies even bred (not that it's a big surprise, I think platies would breed in a mud puddle). Remember when we talked about Ph? Remember how we stated that a lower Ph is said to be acidic? The only way to lower Ph is by adding an acid. In a tank with no Kh, the acid works it's wiles on the Ph which drops faster than the stock market during the Great Depression. Kh resists the acid, absorbs it and goes happily on it's way without affecting the Ph at all. (Yes, I know I'm probably oversimplifying). I stated earlier not to mess w/the Ph of your tank and I'm sticking with it to a point. If you water is too soft then any acids (including the wastes created by your fish) will cause the ph to crash which is not a good thing. If that's your case then you will want to increase your Kh. This can be done by adding sodium bicarbonate (aka baking soda) to the tank, using a crushed coral substrate or adding crushed coral to your filter. As a general rule of thumb, the higher the Ph, the higher the Kh will be. Increasing one will, generally speaking, increase the other. My advice would be not to mess with Kh or Ph unless you have to. By all means measure or call your water company to find out what the values are, but unless it's absolutely essential, I wouldn't mess with it. If it ain't broke, don't fix it and if you don't absolutely have to add it to your water, don't. Great, so where does this Gh thing come into play? When you read books or articles that state that a certain fish prefers hard or soft water, this is what they're referring to. A lower gh means softer water and a harder gh means harder water. Gh stands for general hardness though you might see it referred to as dh (degrees hardness). Both seem interchangeable to me. Here's a chart lifted directly from thekrib.com 0 - 4 dH, 0 - 70 ppm : very soft 4 - 8 dH, 70 - 140 ppm : soft 8 - 12 dH, 140 - 210 ppm : medium hard 12 - 18 dH, 210 - 320 ppm : fairly hard 18 - 30 dH, 320 - 530 ppm : hard higher : liquid rock (Lake Malawi and Los Angeles, CA) Again, it is my opinion and experience that unless water is extremely hard or extremely soft fish can live in anything. If you're breeding, it's a different story, but that's another article. Gh does not affect alkalinity (ph) of the water, but as a general rule of thumb, water with a higher gh will have a higher Ph and vice versa. This is because Gh interacts with Kh and we already know that Kh affects Ph. Whoa, that is a lot to digest there. What do I need to keep track of all this stuff? All you need is a simple inexpensive test kit. I highly recommend a test kit for anyone starting any aquarium of any size. Test kits are cheap ($10-15) and very easy to use. If you're one of those people who is too cheap to buy one, but not too cheap to drive across town to the LFS, you can usually get your water tested free. Avoid the dip strip test kits as they're not horribly effective and stick with the ones that use test tubes and drops. How easy are they to use? Mine uses a needle designed to refill ink cartridges. You suck the water out of the tank, fill the tube to the line that's drawn on it for you, add 3-4 drops depending on what you're testing for, cap the tube, shake and you're done. Very simple. Then compare the color of the water to the color on the card and you've got your reading. If you're filthy rich or just have money you want to spend somewhere (send it to me first), you can even buy an electronic version with a digital readout. My LFS uses these. You stick the probe in the water, push a button and the results show up. I recommend getting test kits for Ph, ammonia, nitrite, and nitrate for sure. You may be able to find out your kh and gh from the water company, but if not, pop for the $5 test kit and skip the McDonald's meal. It wasn't going to be good for you anyway. Wow, now I'm almost scared to own fish. Don't be. It's not that hard. The hardest part about keeping fish is getting the very first tank cycled and running. Now that you've read this article and the article on cycling you're pretty much set. There are other things to think about such as the size and temperment of the fish, but you know the basics of providing a good environment for your pets. Water chemistry is the thing I stressed out about the most when I started and then I realized something. The LFS where I bought my fish kept fish in the exact same water I was using. I asked them how they adjusted their water and they stated that they didn't. They added a dechlorinator of course, but that was it. While I was fretting about how hard my water was and trying to lower the Ph in one tank while raising it in another, I was creating more problems and more stress on my fish. In my personal opinion, dechlorinator is the only thing you need to add to you water if you're not a breeder. The best place to find out what (if anything) you need to do with your water is your LFS. Take them a sample straight from the tap and ask them to test it. Ask them what they do to condition the water at their store. Bear in mind that they have many, many tanks and probably don't have the time or the money to adjust the water chemistry in every tank separately so they're all probably the same. If the fish are thriving and healthy there, then they'll probably thrive and be healthy in your well-cared for tank too.