[Snappy]

Depending on what you are keeping and how heavily stocked your tank is will greatly affect what you need to do, be it water changes or supplemental dosing. I have a heavy load so I go through a lot of calcium, alk & mag.
It's important to get a proper balance. Some important things to pay attention to are:
Calcium
Alk
Mag
ph
Generally natural seawater is about 400 ppm of CA, 7-8 DKH, 1280-1400 MAG, 7.9-8.3 PH.
For aquariums I suggest 380- 440 ca, 7 - 10 DKH, 1350 - 1400 mag, 8-8.3 ph. Again a lot depends on what you are keeping. For example an sps & clam tank will use a lot more of the above mentioned than a softie tank.