Alk is 1st to suspect but than again every other parameter is equally responsible . Sometimes drop in salinity could cause slow problems also. What Alton said has a point too so make u r not directing flow straight in to monties. Also if ur water evels are keep fluctuating like alk and calcium that may be because of not maintaining magnesium. I would deal with ur magnesium 1st and than alk could possibly be corrected on its own . Stability is the key . Temp fluctuations can add a problem too .