Vinegar and lemon, though they are weak and lose buffering potential really quick. What works wonders (and does no harm) is sulphuric acid, obviously very diluted. Battery acid is usually 30% strength, and you can just use a drop to do the trick nicely. In fact, sulphuric acid is used as a long term buffer on many horticulture products to extend shelf-life
Never had to pH up, but if I had to, I'd just use Rhizotonic, it's very strong struff
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