alk keeps dropping off

I check my alk every other day and I am using 2 part dosing. My calcium is steady at 480ppm even when I do a 10% water change and ph is at 8.2 but my alk, I am lucky to get it to 9dkh. Every other day when I test it, it will fall to 6dkh. I have about 80 pounds of live rock and 3 sps corals and 5 softies. I can't believe between the corals and live rock it could be sucking that much alk out of the system.
With the 2 part calcium/alk I am dosing 35ml every other day and I can't fathem that this is appropriate. I used to us a cal reactor but it was very hard to keep the cal/alk at a lower level...{cal500'alk 13dkh} so I put it away. Is there a buffer or maybe a powder that I could use to put in my ro/di holding tank that when I add water it would be adding alk to the system, or is there something that I am forgetting? Also, my mag is 1500ppm

 

njhadfield

your system sounds like it might be an ideal one for Kalwasser
which delivers both calcium and carbonates in the appropriate percentages
and thus can keep up with demand/ utilisation of both
the article below may be of interest for further reading

What Your Grandmother Never Told You About Lime by Randy Holmes-Farley - Reefkeeping.com

hope that helps

Steve