You can use vinegar or citric acids, only problem with those sources is there not stable, meaning if you use them to lower your ph it will creep back up again. Best to use ph down. One bottle goes a long way. Cheers friend
Yeah as said a bit of lemon wil do the trick but do try get hold of PH down. You only dilute a few drops of the bottle before adding to feed and it lasts you a while.