Buffering KH & lowering pH, with no CO2 (yet)

I have not yet had the time to build and install a CO2 generator, but it 
is not from a lack of desire.  In the meantime, I need to decide how best 
to lower the pH while maintaining the KH at around 3 or so.  Until very 
recently, my tank kept experiencing precipitous drop-offs in pH (7 -> 5 
or so) as soon as I made any water changes.  I discovered it was most 
likely due to a deficiency of CO2 for the plants, and that they had begun 
to remove the calcium carbonate from the water to do their photosynthesis 
routine.  Since I have raised the KH from virtually zero to 3-4 (German 
degrees hardness) (using Dupla's KH-generator; I know, waste of money, 
but I hadn't yet learned from all of you about baking soda :)), the water 
has gone alkaline (around 8 pH).  I think David and others have said they 
have used vinegar to achieve the drop.  That's the only method I've heard 
thus far, other than installing a CO2 generator.  Are there any other 
ways to do this?  If not, what is the correct amount per gallon?  Is 
white wine vinegar good for this?  I would assume balsamic vinegar would 
be a waste of a fine ingredient.

Thanks for your ideas.  Mark.