pH.
The suggested reef tank range is 8.3 to 8.4. The pH should hold its own unless alkalinity is low. If alkalinity is OK but pH is low there is probably a buildup of organic acids or a serious lack of gas exchange resulting in the retention/accumulation of CO2 which lowers pH.
Note that it is perfectly normal for the pH of a tank to swing considerably. There is a daily pH cycle where the pH is lowest just after the end of the dark period and highest sometime before the end of the light period. Having a pH range from 7.9 to 8.4 is not unheard of. Larger swings are probably indicative of low buffer levels or poor gas exchange.
Phosphates.
Phosphates, along with nitrates, are a primary nutrient of algae. Tanks with "high" levels of phosphates tend to be infested with hair algae. All authors cite zero ppm PO4 as a good goal. An upper level 0.1 ppm is max
The use of kalkwasser has been closely tied with reduction in phosphate levels. This may be due to precipitation of the phosphates at the kalkwasser injection site, or, more likely, due to increased export via skimming due to the associated higher system pH.
High concentrations of phosphates are found in tap water, which can eventually cause an outbreak of algae
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