My Ph Will Not Go Down What Can I Do?

i just started my hydro and my ph is a little high around 6.5 to 6.8 i need some help i have been adding ph down but it has not worked anyone have any ideas

Take the cap off the pH meter.

i do not have a ph meter i'm using ph testing solution 3 drops to half a vial

what size is your res tank?

keep adding until it goes down, you'll get a feel for the amount.

got it just right thanks guys

My res is a 45 gal but I keep only 30 gallons in it.My ph was at 7.2 when I filled it an drop nutes in.add 1 tablespood of ph minus and down to 5.8 900 ppm (at ec 700) Ec say 1.25 temp is 67 F in tank and things seem to be on a good start now