What is pH?

pH is a scale used to specify how acidic or basic a water-based solution is, in our case, the water itself in our aquarium. Acidic water has a lower pH, while basic water solutions have a higher pH. At room temperature (25 °C), pure water is neither acidic nor basic and has a pH of 7.

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


  • Neon Tetra 6.0 – 7.5
  • Rummy Nose Tetra 6.5 – 7.5
  • Black Skirt Tetra 5.8 – 8.5
  • Angel fish 6.0 – 7.5
  • Zebra Danio 6.0 – 8.5
  • Pearl/ Blue Gourami – 6.5 – 8.0
  • Siamese Fighter 6.8 – 7.5
  • Oscar 6.0 – 8.0
  • Tiger Barb– 6.0 – 8.0
  • Discus 5.5 – 7.0
  • Gold Fish 6.5 – 8.5

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 wild Discus, and certain cichlids, thrive in 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.

Ways to Raise Water pH

As stated above, it is generally a better idea to acclimate your fish to the pH of your water than to adjust your water to suit the pH preference of your fish. However, some people still like to match the natural environment as close as possible or have a pH that is way outside the range that is acceptable to their specific species. If this is the case, there are several methods you can use to raise the pH level in your tank.

Water Changes :

Over time, the pH in your aquarium will drop. The most effective method to raise it back up to the level of your tap water is to simply perform regular water changes. If you do not regularly do water changes, you may need to do several smaller ones rather than one large one (each separated by 24 hours) so that you do not shock the fish by making them go from a low pH to a high one immediately. Vacuuming all of the uneaten food and waste will also help to counter the tendency for the pH to drop over time.

Rocks or substrate:

Add some rock work or substrate to the aquarium that has the effect of raising the pH. For example, crushed coral is used as the substrate in many African cichlid tanks (African cichlids prefer a high pH). Limestone and petrified coral will also do the trick. If you do not want to add these rocks to your aquascaping, you can add a bag of crushed coral to your filter or hide some of these rocks behind the rocks you do want to showcase. Be very careful when using this method, however, because it could raise the pH in your tank beyond the appropriate level.


Increasing the oxygen concentration in your water will serve to drive down the carbon dioxide concentration. As discussed above, less carbon dioxide translates to a higher pH.

Therefore, you can increase the aeration in the tank to raise the pH.

Baking soda :

Adding baking soda will raise the pH, but remember that this will need to be constantly added (you cannot just add it once and forget about it). You also need to be careful not to add too much at one time and cause a severe spike as this could kill your fish. It is best to gradually adjust the pH if you decide it must be adjusted. A general rule is 1 teaspoon per 5 gallons. Dissolve the baking soda in some water before adding it to the tank. Also remember that the above ratio of 1 teaspoon per 5 gallons is just a rule of thumb. For your specific case, take it slowly so you do not shock or kill your fish.

Removing Anything that May be Lowering the pH:

Below is a list of ways to lower the pH in your aquarium. Study this list to see if you are accidently using any of these methods and getting a low pH as a result. For example, maybe you placed a piece of drift wood in your tank without realizing it would lower the pH. If this is causing you problems, remove the drift wood.

Ways to Lower Water pH

Lowering the pH in a freshwater aquarium is often more difficult than raising it. There are some methods you can try though..

Filter Through Peat Moss:

Filtering through peat moss is the most effective way to lower your pH. Some people also use peat moss in their substrate for the same effect.

Add Carbon Dioxide :

As we have talked about multiple times, increasing the carbon dioxide in your tank lowers the pH. Therefore, pumping in more carbon dioxide would result in a lower pH.

Add Wood :

Many types of driftwoods will lower the pH. To read more about using wood in your aquarium read the article on this site entitled Adding Rocks and Wood to Your Freshwater Aquarium.


There are many products on the market today that will lower your pH. However, just like the chemicals that raise the pH, these pH lowering compounds do not maintain a stable pH.