My alkalinity is always low and I never have a problem. It's been like that for 14 months and I have a mixed reef with sps, lps and soft coral. All are healthy and grown fast. My alkalinity drop sometimes around 5, usualy is around 6. sea water is around 6.5.
Here is my tank...as you can see I have quite a mix of coral. I dose for alkalinity and calcium but I keep the alkalinity around 6 anyway. I use the Bulk Reef supply soda ash sodium carbonate as it is pure and I trust it has no additive.
http://www.youtube.com/watch?feature...&v=Mv2G00XjCh4