The washing soda is a little tough to find; I never did find it, but I'd like to know where it is if you find it. Washing soda is sodium carbonate; baking soda is sodium bicarbonate. Since all this dissolves to carbonate ions in the tank, you can use straight, plain baking soda as a buffer, provided you are not trying to raise your ph. The reason people add washing soda is because it will quickly raise your ph; I'm not sure why exactly, I suppose it has to do with the reactions that occur as the stuff is dissolving.