We learned in school that water, or H2O, is composed of hydrogen and oxygen molecules. Neutral water is given a pH value of 7.0. It contains equal amounts of hydrogen ions (H+) and hydroxide ions (OH-). Dissolved chemicals and minerals change the balance of those ions from a perfectly neutral state.
Increase the number of hydrogen ions (H+), and the water becomes more acid ("low pH"). Increase the number of hydroxide ions (OH-), and the water becomes more alkaline ("high pH"). The further these values rise or fall, the more acid or alkaline the water becomes.
What Is Normal pH?
There is no "normal" pH that applies to all fish. Because fish originate in ponds, rivers, streams, lakes, and oceans that have different pH levels, their needs are different. Saltwater fish prefer an alkaline pH of 8.0 or above. Freshwater fish thrive in a range lower than that, somewhere between 5.5 and 7.5, depending on the specific species.
Keep in mind that pH is not static, it changes over time; in fact, it even changes over the course of a single day. Typically it drops at night and rises during the daytime. The pH will change as new fish are added or removed, as water is added or changed, and as the biological processes change in the tank.
Preferred pH of Common Freshwater Fish
How Important Is pH?
Changes in the pH, especially sudden changes, can prove harmful or even fatal to fish. As the pH rises, it increases the toxicity of chemicals such as ammonia. It is an important factor to monitor during the break-in of a new tank. pH changes are particularly hard on young and sick fish. In a number of species of fish, breeding occurs only within a specific pH range.
If you are planning a new aquarium it's wise to know the pH of your water source, so you know beforehand if it is compatible. Some fish such as Discus, and certain cichlids, thrive in very narrow ranges of pH which should be taken into consideration when setting up their aquarium.
When moving fish from one aquarium to another it is important to match the pH levels. Sudden changes in pH account for many fish losses that occur when fish are brought home from a pet shop. Neon tetras are particularly sensitive to sudden changes in pH, and can easily be shocked when moved.
How Often Should I Check pH?
pH should be tested at least once a month, preferably every two weeks to allow for detection of trends before they become a problem. Test results should be kept in a log book for future reference. Remember that because pH can vary based on time of day, testing at different times of day can yield different results even though nothing is wrong. For this reason, testing should take place the same time of day, preferably in the afternoon.
Any time there is a fish illness or death, the pH should be tested. If the tank is treated with medication, the pH should be checked when treatment is begun, on the final day of treatment, and again a week later.
It is also wise to test your water just before purchasing new fish. Check with the shop where you are purchasing the fish. It's important that the pH of water the fish is currently in is not significantly different than the pH of your water at home.
Should pH Be Altered?
I recommend sticking to the axiom of "if it's not broken, don't fix it". Don't spring into action simply because the textbook says the optimum pH for your fish is 6.4. and your water tests out at 6.0. As long as the pH is stable, and the fish show no signs of distress, it's best to leave the pH at its natural level.
If the fish are not thriving, or if testing shows that a trend is occurring, such as a steady drop or rise in pH, the problem should be addressed. Proactive water care is always your best bet. Performing frequent partial water changes, and vacuuming the gravel are the most important things you can do to keep water pH stable.