Alk drops like a rock - any ideas?

Bad test kit maybe?

 

Low Mg would be my guess, w/o proper Mg levels keeping CA and Alk in balance is tough to do....
Another option could be the water changes, whats the alk of your premixed saltwater. 
Are you dosing with BAKED baking soda?  Or just Baking soda?

 

I would double check kit as well, I would get the mag up a bit as that will help balance things.

How big of a drop are you getting in what period of time? How fast does calcium drop?

If the calcium isn't dropping as fast there is something more than just coral growth happening. My first hunch is excess Co2 in the system driving down your Ph. Adding regular baking soda will help with Alk but will lower Ph, try baking it at 350 deg for 45-60 min, that should get the Co2 out of the soda allowing it to raise Ph when you add it.

How are you maintaining these values? 2 part?

Adding kalk will raise both Alk and Ph so that would only add to these numbers.

 

I think dripping Kalk may be an easy solution and if your not dosing anything currently now is a good time to start. Agreed with the Mg needing raised a bit to help stablize but as far as getting Alk up and keep Ph between 8.2 - 8.4 then get to dosing my friend.

There is a product called Kalk+2 that worked well for me in a simple gravity Kalk dripper. It is kalk with added Stronium and Mg and when I used it I never had to dose Mg as this kept it in good range along with regular water changes.

Im no professional and dont have the knowledge and experience alot of the guys here do so they will have good advise BUT I know what worked for me. Hope you get it figured out soon b4 any damage is done. If your color is already gone on your corals then we need to get your tank stable asap.