I don't think your system's consumption of alkalinity is out of line. Keep in mind that the consumption of bicarbonate ions as well as the utilization of CO2 as a byproduct of animal respirations is the primary source of the Carbonate in stony
coral skeletons. Not only is alkalinity responsible for maintaining the pH of seawater, but it is also half of your supply of minerals to the sinks represented by coral biomass. Photosynthesis has an effect on this as well. During the day, photosynthesis is higher than respiration, resulting in a net consumption of CO2, raising the pH of seawater in systems where there are corals, the greater the number of corals/photosynthesis, the larger the consumption of bicarbonate ions:
Ca++ + 2HCO3- --> CaCO3 + CH2O + O2
Whereas at night when photosynthesis stops, the 2 equations combine due to continued (although at reduced rates) calcification and respiration of corals and macroalgae:
Ca++ + 2HCO3- + CH2O + O2 --> CaCO3 + 2CO2 + 2H2O
So that during the night, some of the bicarb is consumed to make the latticework for
coral skeleton (boy, deep subject here) and prepare the calcioblastic tissue for the deposition of CaCO3 during photosynthesis. The stoichiometry of these two reactions is further skewed due to the buffering effect of seawater and photosynthesis of plants/macroalgae, such that the buffering capacity in a
closed system becomes a consumable item, rather than a balanced product of the open sea. As calcification/skeletalization occurs in corals and coralline algae, alkalinity will be consumed until it reaches a level where it becomes a limiting factor in the growth of those organisms that consume it. This does not take into consideration the loss of alkalinity due to neutralization of organic acids that are the byproduct of decomposition or animal respirations (fish, benthic infauna, bacterial respiration, etc.)
If you are using up the buffer at the rate you have stated, I would not be alarmed, but elated, as it reflects a measure of your coral growth. I would only be alarmed if your pH were to stay below 8.0 24 hours a day, then you'd have problems. I would stay the course.
Hope this helps, Lemmeno if you need more info on the calcification info, bicarb is an essential to this process. There is a good review article by McConnaughey and Whelan in Earth Science Reviews (1996) that gets into some of the newest thinking on skeletalization and calcification in corals, pretty good review article if you can get it.
