I try to keep mature plants at 24" or less ,to make better use of the lighting,depending on the strain,i induce photperiod at about 6-8",I veg at 24HRs,I dont use 18-6 because it promotes internonodal stretching,I prefer short squat bushy plants,a 400WT lamp will only penetrate about 6",so the taller the plant ,the light to surface ratio decreases,giving you a smaller harvest for the longer growth period,as for the pH,start at 5.5,and do not let it go over 5.8,the pH will rise as the plants feed ,so just maintain at 5.8 or slightly under till the next res change,use a pH down solution for this,using nutes to lower the pH can cause an overfert,an EC meter can help too,though i dont use one,i can pretty much tell what the plants need by observing them,but i've been at it a while....bonbon http://www.cannabisculture.com/articles/3127.html