A few quick words for you Liverbird. Nitrites will not cause a rise in pH. On the contrary, nitrites are often present in the nitrous acid form and will tend to drive your pH lower. Do not fret too much over the pH of your tank water. As long as you can keep the ammonia and nitrite levels good, the fish will adapt to a wide range of pH in fairly short order. In a natural environment, the simple plant growth that takes place in a lake or brook will drive the pH all over the map daily. At night, no oxygen is being produced from photosynthesis and the respiration of plants and fish will raise the CO2 concentration resulting in a big drop in pH. When the sun comes out in the morning, the plants will quickly consume all of the available CO2 in the water and the pH will rise quite a bit, often a whole unit, like from 7.0 to 8.0. The fish simply go on with their lives and ignore the change. Many of the rapid changes we see in our tanks pH from tap water are merely changes in dissolved gasses. That is something the fish will ignore just as they would in a natural water system.
If you are worried about the source of the pH change, try a little experiment. Draw a sample of tap water and read its pH, then read the same water sample's pH a day or two later. I would bet that you will find that simply letting the water sit around in a glass container and lose its dissolved gasses will show a large pH change after a day or two. If it gets close to what you are seeing in your tank, there is nothing at all to find in the tank. The pH change is a simple result of gassing off fresh tap water.
