low alk (6dkh) and calcium 400ppm ???????

[bigfish]

I'm cycling a tank, 20g, for 2 months now. I got matured live rock for another reefers tank and threw in a yellow tail damsel a month ago, hes doing good. Now I'm noticing a drop in alk. how can i get this back up, other than the obvious 10-20% water change. I think i herd somewhere that baking soda could raise alk? I am using tap water distilled for 24hrs (i know i should change, but all in good time). Anyone?