I have found that a pH below 6.6 retards bacteria growth and function, as does low dissolve oxygen. By keeping the pH between 6.7 and 7.3 on the outside helps keep the system operating at its best. By doing so, I have found that not only does the ammonia and nitrite levels remain low, but the heterotrophic bacteria seem to do a better job in mineralizing the fish waste solids. If I let the pH get down around 6.5 or lower, the fish tank water starts turning yellow, fish waste solids accumulate in the bottom of the tank and the chemistry is less than ideal.

According to the folks at UVI, the best way to keep the pH up is to add Potassium Hydroxide (KOH) and Calcium Oxide (lime) to the water on an alternating basis. Due to the high calcium in our water, I have yet to add the lime but I do add the Potassium Hydroxide on a weekly basis. If you add Calcium Oxide it will combine with the water to produce Calcium Hydroxide. Both the Potassium and Calcium Hydroxides are bases, which will raise your pH. Be sure to mix them with lots of water prior to adding them to your tank and go slowly so as not to harm your fish.

Potassium Hydroxide can be purchased at you local Hydroponics store as pH-UP. Be sure to look on the label as there are pH-UP products that contain other chemicals. Don't use them. In concentrated form, it can be rather nasty to handle so use gloves when mixing and rinse the outside of the container before putting away for the next use. Lime powder can be purchased at Home Depot in large bags. I haven't used the lime I purchased yet, so I can not vouch for its safety.

Oliver