yes.its recommended to flush with 3 times the amount of h20 to pot size.1 gallon pot =3 gallons water.i wouldnt go that heavy though.if its toxicity you should see your new top growth lighten pretty damn quickly after your flush.like overnight.as long as your soil isnt too heavy,it will dry out and overwatering should not be a problem.
is everything super dark green with slow growth? post some pics