I wouldn't add baking soda. Ph is something you really don't want to mess with unless you have to. I had a problem with my water, that I "think" was contributed to soft water a few years ago, so I usually have a bag of crushed coral in my tank to saftely raise the ph, and buffer the water. I would avoid messing with your ph if you're not having problems, but if you really want to raise it, crushed coral has always worked for me. I would not use baking soda. Just to add some tips for using crushed coral if you're going to, try to match the ph of the tap when it first comes out, that way there won't be any ph fluctuations when you do waterchanges. For instance, in my tap water I have a ph of 7, but after a few days that drops below a 6. If you have a ph of 6.4 out of your tap, I'd still try to match your ph in your tank to a 6.4 instead of the supposed "ideal" 7.0.