While necessary for some things such as bacterial reduction of nitrate to nitrogen, these areas can also produce hydrogen sulfide and other undesirable substances. It's a form of CaCO3. These are available in several varieties, such as tar, transformer, and electronic. They must be properly matched to the type of bulb you are using. These filters make use of bacteria to break down wastes in the water into substances which are less toxic to the inhabitants, in a process known as the nitrogen cycle .
Examples are under gravel filters , sponge filters , and trickle filters .
A number of species prefer partly-salty water. They are easily hatched and their eggs may be stored dry for years. Buffers may be formulated to adjust the pH to a particular value, or to raise the alkalinity without changing the pH. calcium chloride CaCl2. This is a form of calcium that may be added to reef tanks to maintain the calcium level. See kalkwasser .