Quote:
Originally posted by Geoff
...i seem to have a problem keeping my ph up at night...
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prolly partially to blame, as Doug has already mentioned that there is surface degassing of CO2 at night, which will drive your pH down if it is not exchanging with the atmosphere (increased pCO2 in the water column drives pH down).
However, there is a normal swing every 12 hours in pH in both our tanks (more pronounced) and in the ocean based on utilization of CO2 from the water column by photosynthesizing organisms. By utilizing the CO2 to make carbohydrates, these organisms effectively reduce the net levels of CO2 in the water column, at least at a rate faster than atmospheric diffusion can resupply it (whooo, LOTS of mathematics and chemistry to demonstrate this, lets leave it for another time
). If you have significant amounts of coralline and/or macroalgae, then you will see this swing on a daily basis. At night when the lights go out, these same organisms turn off the photosynthetic machinery and become net producers of CO2 (they "burn" the CHO's at night for energy from what they have been making all day), so that at night, not only do you have animal respirations producing CO2, but there is no longer the removal of CO2 for photosynthesis AND these same creatures are adding their own net CO2 production. This gradually drives down the pH of the water column during the night until the daily nadir is reached after dawn, when the photosynthetic reactions get enough light to restart for the day.
Symbiodiniium spp in corals contribute to this phenomenon as well, although most of their CO2 production is utilized internally by the animal tissue at the calcioblastic endothelium if you subscribe to Tim McConnaughay's or the Goreau calcification models.
In other words, you should see a diurnal swing in pH, with the lowest levels in the morning (7.95 to 8.0) and the highest levels in the evening (8.4 to 8.6). The amplitude of the swing will depend on the populations of your tank, the amount of water in your microcosm, the intensity of the lighting, your alkalinity and calcium, the depth of your sandbed, the surface area of your tank (and whether you have an open or closed hood) and the circulation in your system. If you have poor exchange of indoor air in your house with fresh outdoor air, there may be a net increase in the CO2 levels in the air above the tank (as does the use of glas covers), leading to excessively low pH in the mornings and a depression of pH during the photoperiod. Use of a Ca reactor that is out of adjustment may lead to sustained lowering of pH as well. Many factors to consider.
Hope this helps explain what may be happening.
). There may be an occasional conditional drop or increase outside this range, but prolonged pH values outside this range are signs of impending doom, and usually can be seen in the specimens in the system. 
