Kamla- In answer to your question about why the PH swings...
The PH of properly buffered seawater (alk of 2.5meq/l or higher) is entirely dependant on the amount of CO2 in the water. The more CO2 in the water, the lower the PH. When the lights are on and corals and algae are photosynthesizing, they are consuming CO2 and emiting O2. This depletes the CO2 to some extent and raises the PH. When the lights go off, the algaes and corals respire just like animals do, consuming O2 and emiting CO2. This increases the CO2 in the water, lowering the PH. One may also notice decreased PH in general in the winter months, as the CO2 levels indoors tend to rise in a well sealed house, while open windows in the spring/summer/fall tend to keep CO2 levels indoors closer to normal air levels.
To counter the nighttime drop in PH, there are a couple of strategies you can employ. You can add a refugium with macroalgae that is lit on an opposite cycle from the main tank. This way the algae in one tank is photosynthesizing (and consuming CO2) at the same time that the algae in the other tank is undergoing normal respiration (creating CO2). If the tanks contain an equal number of photosynthesizers and equal lighting, they will exactly balance, creating a rock steady PH. That's pretty hard to do in practice, but you can come close enough to limit the swing to .1 or .2. Another way would be to drip kalkwasser at night. The kalk uses CO2 to convert from calcium hydroxide to calcium and carbonate, raising the PH.
