75ml of PH down???? Holy Shit!! Your tap water has to be extremly high. I have tap water at 6.3-6.4 and I have to put in 2.5 of ph down in 12L of a 5 gallon bucket. I use liquid PH down and 1 ml in 12L will bring down the PH .2.
wowie, I'd try another botle of pH down, maybe it's freaking old. make sure you check pH after you add nutes to water not before. most nutes lower pH some themselves