Baking soda. The chemistry behind it boggles me (NaCO2 will bring your pH to around 8.0 and will boost your alkalinity... Higher alk means a drop in pH is resisted.. So how does that work?), but I've heard of many RC people using it.
Muriatic acid is another common chemical used. Personally I'd just let it be. Without your rock to buffer it, your pH would plummet to 5ish on its own.. It should plummet in time to the pH that the rock buffers to anyways.
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