*Disclamer: I am not a chemist I am a fish keeper (although you have to be both sometimes) and here is my depiction of pH the way I understand it. Always do your own research *
The story: I had moved from Washington living off a deep aquifer to the middle of Idaho surrounded by agricultural land. I have always known about pH but never really understood why it was important. I had bred my fish as usual and everything was going fine until I moved them into the growout tank and then they started dropping like flies. I also noticed a few adult fish who had a water change around the same time also perished. Confused and concerned I did more water changes not knowing I was doing more harm then good. I tested my water but never really took note of the pH I knew it was high but betta are good adapters what I didnt know was that the pH had been swinging wildly after it rained due to chemicals in the run off and had silently been killing my fish.
I have been keeping betta fish for many years other species of freshwater fish for many many more. I never really put much thought into pH until recently and was kind of shocked to notice nothing in depth about pH here. Betta fish and other fish in general can tolerate different levels of pH but until recently I learned how important it was to monitor in tanks. I am hoping if you have never really put that much thought into your pH then you may be able to learn from my mistakes.
Many of us know about pH. We understand that freshwater fish are about 6.6-7.4 depending on the species. But I will try to explain everything I learned from the beginning. The term pH stands for potential Hydrogen and when equal or neutral the reading is 7.0. pH is a measure of acid and alkaline (or base). The scale on a scientific measure is from 0-14 with each raise ( for instance 7.0-7.1) being a ten fold difference in the amount of hydrogen.
Let me through a little math at you...
pH=-log(H+) pOH= -log(-OH)
If both equations are equal you are neutral in the pH scale at 7.0. This equation helped me understand but if its still fuzzy keep reading...
A increase in hydrogen ions +H turns it more acidic while a increase hydroxide ions -OH, turns it to the base. Now this is all fine but your probably wondering how it affects your fish. More acidic water has entirely different chemistry how it acts on metals and other particles in the water, where as more base and different things dissolve (Meaning the pH can effect how toxic metals are to your fish, which is rather important and can affect how medicines work on your fish). These dissolved particles help make up your buffering capacity (water Hardness). Which is also important because a higher buffering capacity the better the water is at maintaining a pH level. Simple rule to follow is soft water less buffering capacity, hard water more buffering capacity. There are many little ions that float around waiting to buffer your water calcium, phosphate, magnesium, nitrates (nitrate dose not have much buffering power but can direcly affect pH another reason to clean your tank) , etc.
One quick thing to make clear: "There are 2 types of hardness permanent and alkalinity. Alkalinity or temporary hardness is usually done by calcium and magnesium ions and permanent harness or buffering is usually done by bicarbonate and carbonate ions. However if you didnt understand that much its fine because most buffering and hardness are very closely related due to the fact that hardness is usually formed by calcium magnesium carbonates." I recommend reading more here http://www.cichlid-forum.com/articles/hardness.php
I always loved this example, if you dont clean your aquarium for a long time forget the ammonia build up for a second, it can lead to an increase of phosphate which can in turn affect the number of phosphate ions floating around which can bond with calcareous material precipitating calcium from your aquarium which reduces your aquariums ability to keep pH stable. ( not to mention phosphate leads to algae blooms, and buffers pH to lower levels sense they are buffering ions, etc.)
pH is not a static entity it changes over time and dose fluctuate the idea is to keep the pH as relatively similar as possible and keep spikes from happening. Say you forget to check pH (like myself) and add water from the tap into the aquarium. If you add water with a pH about 8.0 with a high buffering capacity, to a tank with water about 6.8 even if its just a 1/4 water change your pH will quickly spike to near 8.0. and to get it back down you would need to do more then the quarter you originally did because those particles are now in the water raising any lower pH you might add back up to 8.0. The idea is to avoid these spikes because these can cause serious harm to your fish.
Here is a wonderful chart from http://www.state.ky.us/nrepc/water/wcpph.htm
Limiting pH Values
3.8 10.0 Fish eggs could be hatched, but deformed young are often produced
4.0 10.1 Limits for the most resistant fish species
4.1 9.5 Range tolerated by trout
--- 4.3 Carp die in five days
4.5 9.0 Trout eggs and larvae develop normally
4.6 9.5 Limits for perch
--- 5.0 Limits for stickleback fish
5.0 9.0 Tolerable range for most fish
--- 8.7 Upper limit for good fishing waters
5.4 11.4 Fish avoid waters beyond these limits
6.0 7.2 Optimum (best) range for fish eggs
--- 1.0 Mosquito larvae are destroyed at this pH value
3.3 4.7 Mosquito larvae live within this range
7.5 8.4 Best range for the growth of algae
Overall there are many things (ions) in the water that can change your pH the idea is to monitor the pH and keep track of it. I hope that I could lead you into learning and understanding more about pH and tell you why it is important to keep learning about something that is sometimes overlooked.