The ideal pH level for an oscar fish is 7.2. Levels either over or under 7.2 (between 6 and 8, according to Aquarium Life) should not be a big problem because oscar fish are fairly adaptable, just so long as the change is gradual.
Maintaining Proper pH Level
An oscar fish can adapt to different pH levels, provided those levels change gradually. Anywhere between pH levels of 6 and 8 are acceptable, though 7.2 is the ideal. The key is to maintain a continuous pH level, even after water changes and cleanings. Whatever the pH level is for the water the oscar fish is swimming in, your oscar needs the level to stay that way, so maintaining a continuous pH level is key to your Oscar’s health.
Add an acid to the tank water if you want to lower the pH. Add a base if you wish to increase the pH. However, regulating pH in this way becomes more difficult if your water contains buffers. Buffers will cause the water to maintain a high (or low) pH. Buffers further complicate matters when you change your tank water, because the new tank water must match the pH levels of the old, existing tank water, and this will require a careful mix of buffers and pH adjusters. Some people recommend adding a piece of wood to your tank to lower the pH, or adding a seashell or a coral skeleton to raise the pH as easier, “softer” methods of adjusting pH levels; be aware that these methods will take longer to produce noticeable effects.
Performing frequent partial water changes and vacuuming the gravel are effective ways to keep water pH stable.
Testing pH level
Use a tank testing kit. These are available from any aquarium supply store, and usually include guides for proper use and testing. Take a sample of your tank’s water from about 8 to 12 inches below the water line.
Importance of Proper pH Level
As already stated, a pH level of 7.2 is ideal for an oscar fish, but it is not necessary. Oscar fish are adaptable. Don’t panic if you test your tank and find the pH level at 6.4 or even 7.8. It will be important to take note of this level. Observe your pet for some time to see how it is adjusting to the new pH; the most common problems associated with improper pH levels are skin diseases. If you do not see any apparent negative effects to the current pH level, make it your goal to keep your tank water at or near that level over the long-term. Short-term fluctuations in pH level are to be avoided whenever possible.