what is your calcium.
when you raise Cal you must also adjust your Alk. Alot of people use B-ionic, BI(B-ionic) comes in 2 parts A & B, A is for adjusting calcium, B is for adjusting the alk to the same lvl of calcium, so if you add the sma eamount of part A & B your cal/alk will be in line....
But if you dont have SPS corals (small polyped stoney) then you have something wronge with your magnessium, SPS corals make up a skelton of Calcium carbonate, so they suck your calcium out of the water which makes you have to constantly adjust it.
If you have SPS or lots of LPS -
Buy B-ionic and keep your cal/alk lvl in range
At this site
http://www.reefnerds.com/ChemPlot/ChemPlot.html there is a java programe that you can log your Cal/Alk lvl in and see if they are in range of natural water.
If you dont have SPS or lots of LPS -
Go to your LFS and have them test your magnessium/or you can buy your own kit
Mag should be around 1300.
when i was starting off i had a real prob with my mag, it was around 700, i was using instant ocean salt, i did many water changes 20% 30% even a 70% when i was desperate (mind you i had no corals, just some fish) and it when up a tiny but!
then i switched to Oceanic salt and now its perfect!
Sorry if i made my post sound like you where stupid, but you realy didnt give enough info on your tank.
1. How old is you tank?
2. What corals do you have?
3. What salt do you use?
4. How often do you do water changes?
5. How high is your salinity?