You can buy baking soda anywhere. Supermarkets, Walmart, even the Dollar Store. A box costs $0.99 and is perfectly safe for your reef. Baking Soda is sodium bicarbonate which can be used for baking, cleaning, and deodorizing. And we hobbyists use it to buffer the alkalinity level of our aquariums, too.
After making myself (literally) do all my water testing today on both systems, I'm quickly reminded why I prefer to only run one reef at a time. Those people that run multiple tanks or frag systems deserve heavy praise for being able to keep so many separate systems thriving. I noticed some bits of corals turning white in my reef, but I kept blaming it on shade rather than a water quality issue. Today, I ran through all the Elos test kits, twice since I have to measure the 400g reef as well as the 60g frag system.