Sunlight and bright aquarium light effects pH. At nighttime, plants stop taking up carbon dioxide and use oxygen (O2), when this happens, the pH can drop. During the time the sun is shining or bright aquarium lights are on, the pH can rise several tenths of a point as the plants take up CO2 and give off oxygen. The bright light on aquarium plants can help reduce and control nitrate, making the pH more stable.
Hardness is generally thought of as being the elements of calcium and magnesium, but there are many others that add to hardness. Calcium (Ca++) and magnesium (Ma++) are the major elements we normally look at as the contributors to tap water hardness. Distilled or pure water has a hardness of 0 ppm.
Aquarium test kits measure the water hardness in ppm (parts per million), or German DH (gDH). Germans have a large influence in the aquarium hobby so some test kits measure in German DH (sometimes “General Degree of Hardness or General Hardness (GH)”). The international standard for expressing hardness value is ppm or its equivalent milligrams per liter (mg/l).
The United States Geological Survey (USGS) provides general guidelines for classification of waters: 0 to 60 mg/L (milligrams per liter) (same as ppm or parts per million) of calcium carbonate is classified as soft; 61 to 120 mg/L as moderately hard; 121 to 180 mg/L as hard; and more than 180 mg/L as very hard. Parts of Southern California are reported to have greater than 1000 mg/L.
One degree of GH (gDH) equals 17.86 ppm (mg/L).