The bacteria will slow down some at very high pH values but what choice do you really have. I'm glad to hear you are aware of the minerals you have as decorations. Most of us have to struggle along using crude measurements of actual pH effects of our rocks to decide whether they are safe to use. I must agree that a silicate is very unlikely to cause any effect on the pH of the water. One of the tricks tried by many when they have a low carbonate hardness is to use various wood decorations. The wood tends to leach tannins into the water which are slightly acidic and will lower pH of poorly buffered water.
now i feel stupid 
i put 6 drops in ...instead of 4!
ph is back around 7 now!
