The range goes from 0 - 14, with 7 being neutral. Since pH can be affected by chemicals in the water, pH is an important indicator of water that is changing chemically. Water with a pH of 5 is ten times more acidic than water having a pH of 6. The acceptable range for most fish is from 6.5 to 9.
Carbon dioxide acts like an acid in water.
Carbon dioxide is used during photosynthesis. So when photosynthesis is not occurring, pH has a tendency to decrease at night and increase during daylight with photosynthesis occurring. The most important thing to remember about pH is that you want to avoid wide fluctuations. When stocking fish, it might be necessary to acclimate the fish to the receiving pond. It is recommended that for each 1 unit difference in pH, temper fish for 20 minutes.