Using PH Down

I have stunted my plants because my soil had a too high of a ph 7.8. I have used lemon juice to get it down to 7. I plan on using sulfuric acid to make a ph down solution of 6.0, does this sound like a good number? another question how often should i use it.
If i get the ph to the right level do my plants have a chance of rebounding. they are 2 weeks old but look only 4-5 days old. they are blue mystic autoflower and I have spent a lot of money on seeds. please help. I have learned my lesson on using cheap soil. Is their any decent soil I could find at a big box chain.
 
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