In order to lower your PH you might want to consider buying a "PH Down" solution. How many gallons of water are in this bucket? Does it recirculate? Be careful with the solution, you will probably only need a drop or two. If you go to low don't add "PH UP" simply pour some water out and replace with tap water and try to PH it again. I typically try to keep my PH @ 5.8 in order to allow nutrient availability.
Get some ro water from the market, use that water, ph it to 5.8. Lower water level to just under net cup, not touching it. Make sure your stone aeration is turned up high, lots of bubbles under the net cup. Wait. Keep an eye on ph. Do only 1/3 nutrients.