Baking soda will lower pH. Washing soda will raise pH.
The Reef Chemistry Calculator to help determine how much to add:
http://home.comcast.net/~jdieck1/chem_calc3.html
Aquarium System Volume Calculator to better determine the actual water volume in your system:
http://home.comcast.net/~jdieck1/volcalc.html
As well as...
High pH: Causes and Cures:
http://reefkeeping.com/issues/2005-03/rhf/index.htm
Low pH: Causes and Cures:
http://www.reefkeeping.com/issues/2004-09/rhf/index.htm
Dominik,
That macroalgae almost looks like some kind of caulerpa, especially if it is invasive.