stability

OK I'm new to sps corals but I've been reading everything I can about keeping them k here's my issue I bought 2 montiporas and a birds nest and they all frags my tanks been up and going for about a year my pH is around 8 my cal was always at 420 mag 1200 and alk 8 as for nitrates I kept at 0 to .05 and 0 nitrite and 0 phosphate tank is a 29 gallon with 56 pounds live rock shallow sand bed using a coralife super skimmer 65 gallon with seachem matrix in the diffuser box for lighting I have a odyssea 30 inch metal halide 250 watt with 4 24 watt t5 ho antics run my halide for 4 hours and antics run for 10 OK now for my question my cal and alk are always dropping day by day so how do I keep it stable at all times I use kent nano reef a and b but my cal and alk don't stay constantly at one level