ok when I first strated adding B-Ionic for the first month I got my calcium up to 550 (didnt have a test kit at the time,now I do) then lowered it down to 450. it stayed at 400-450 for about 4 weeks now no matter how much of the B-Ionic I add I cant get it up over 350.
Raising your Cal will lower your Alk and vice a versa. If you are having trouble raising your Cal try checking your Mag level. Without getting into a chemistry lesson ....it does effect the amount of Calcium that can be held in solution. Usually a good water change with a quality salt should raise your mag level, but if your level is that low you may have to dose magnesium.
Check your sump and other equpment for calcium buildups. That was my problem before. My sump looked like a bunck of dead corals cuz the calcium was just building up in there. Once I cleaned it out, everything was back to normal.
Let us know what your alk level is. I can't really determine what your problem might be without knowing more about your water chemistry. Also, go ahead and do a mag test--either buy a test kit or have the LFS check the level for you. Then we'll be able to come up with a cause.
CO2 shouldn't be a problem assuming he's running a skimmer.
I second Alk and Magnesium tests. I don't believe that magnesium would be much of a problem unless A) there is a severe lack of water changes or B) he's using a crappy salt but then again, are there even crappy salts on the market?
W/out any other info I bet that his alk is high, from adding too much buffer to top-off, etc, and it's driving his calcium down. Any takers?