alk and calcium problems

I am having major problems keeping my alk and calcium up! I have been adding buffers everyday for the past 3 weeks and they still wont stay up overnight. I have been adding Seachem reef bulider for the alk, and Seachem reef advantage calcium to raise calcium. Both of the readings are as follows- KH/alk 7.0-2.51 meq, calcium is always at 370 ppm. I use Salifert test kits and have them checked at the LFS. My tank only contains two SPS a boat load of zoanthids and a monster toadstool. The tank has been up for years and I do a change about every month, whenever the trates get to high, with Coral Life salt. Any ideas would be great because I am witts end.

 

bump, and PH is 8.3

 

AH! I just read a thread on magnesium, that might be my problem. No LFS in all of Saint Louis sell them!

 

Magnessium levels could certainly be the problem, but test first, if low you can use epsom salts to raise the Mag level.

John