As Mr Limpet said lime works but be very precise in the type of lime you buy,you want Dolomite or Dolomitic lime to lower ph,most garden lime products are made with dolomite lime as well but still read the lable,some are not made with dolomite lime & will raise the ph,also using lime to correct ph issues is a long term solution,its not a solution that shows quick results.
Also as Limpet said you can lower the ph levels by adjusting the levels in the water,this course of action gives faster results,next time you start a grow try adding the proper amount of lime to the soil before you plant,this makes for proper soil ph all the way through the grow.