Optimum Freshwater pH

for the week of 10/29/98

The one bit of aquarium chemistry that almost everyone has some familiarity with is pH, which basically is the measurement of how acid or base a given solution is. As most will recall, a pH value of 7.0 is neutral, while higher values indicate alkalinity and lower values acidity. In nature, tropical fish might come from waters with quite low pH values (e.g. Discus and Tetras from the Amazon River Basin might be collected at a pH of 5.0) or very high values (some African Cichlids might be found at pH 9).

Since pH is a familiar concept and is easily tested, many beginning hobbyists overemphasize its importance and go to great lengths trying to achieve some “Optimum pH Level,” most often aiming for the neutral value of 7.0. This is quite often unnecessary, and can cause problems.

Generally speaking, the optimum pH for a freshwater community tank is whatever the pH of the supply water is. Water with a pH value between 6.5 and 8.0 is acceptable to keep and even breed most common tropical fish. And in the cases of the few fish that require lower (Discus) or higher (certain unusual African Cichlids) pH levels, other factors such as Water Hardness and Total Dissolved Solids appear to be much more influential than the pH itself.

Sudden changes in pH, on the other hand, can be very stressful to fishes – even if the values remain in the recommended range. Since many local fish stores make the fish adjust to the local pH, the aquarist who artificially adjusts his pH may actually be causing undue stress, rather than relieving it. In addition, tap water is sometimes “buffered,” meaning it contains chemicals that resist pH change by “absorbing,” in a manner of speaking, acids and bases. The addition of a pH adjusting chemical will change the pH for a few hours or days, but it snaps right back, frustrating the aquarist and stressing the fish.

In checking the pH of the local supply water, either let a cup of tap water stand for a day or two, or simply check the aquarium water itself a few days after setup. The reason for the wait is to allow excess carbon dioxide gas, which temporarily lowers the pH reading, to escape. Sometimes water will show a “false” pH of 6.8 immediately out of the tap, but a “true” value 8.0 a few hours later.

If the pH value of your tap water is between 6.8 and 8.0, it’s probably best not to adjust it – especially if your local dealers do not adjust the pH in their aquariums. However, it is still valuable to check pH periodically, to make sure it is staying near its original value. Falling pH usually means that water quality in general is declining, usually because of inadequate water changing. Rising pH is often the result of calcium-based gravels or decorations slowly dissolving.

“Tip of the week” appeared regularly in 1999 and 2000.