alk = alkalinity / KH level.
From the MD site,
http://www.marinedepot.com/ps_ViewIt...product=SF1123
The total carbonate and bicarbonate concentration is also called alkalinity or carbonate hardness and for a stable system the alkalinity should not fluctuate by more than 5% from the optimum level of approx. 2.8 meq/L i.e. a maximum fluctuation of 0.14 meq/L.