To be successful in our hobby you need to dose chemicals. It takes a lot of research to understand the chemistry of seawater and the link below is a good place to start.

The 3 parts of the B ionic system of chemicals is easy to use, safe and very effective. Once you have a good handle on maintaining the chemical balance you may choose less expensive options eg: Kalkwaser and homemade 2 part solutions.

If all you want to do is raise the PH and are not concerned with the other parameters baking soda will work.

http://www.advancedaquarist.com/issues/feb2003/chem.htm