By chance did you check your soils actual ph level before planting? 5.5 isn't terrible for runoff but would be better in low 6's. I had this problem early in my cobra grow my runoff was low 5's to high 4's turned out my foxfarm happyfrog soil had actual ph of 4.5. I ended up taking a chance in week 3 of growth and transplanted into foxfarm ocean forest soil which read at 6.7 soil ph and they turned around within a week. Do you have any ph up solution? You could use the ph up solution to adjust your in water to 7.5 which should make your runoff water low 6ish ph or real close. I know there's other ways to raise and lower water ph(lemon juice,baking soda) but don't know whyich one does which. I'm sure one of the other guys will offer some advice,i think your ok right now if the plant looks good but they'll tell you how to tweak it to make it perfect.