when is a good time to switch light cycles?

[ShuttyMcShutShut]

Im growing a hydro setup in a waterfarm. Ive kept them under 40w flouro at 2" for 24hours a day for nearly 3 weeks now(temp kept at a steady 73). Ive started to use a diluted nute solution(about 1/2 of what GH recommends for their flora series). Im seeing some kind of difficulty there as some leaves are yellowing and spotting(ebb and flow cycle= 1on/ 2 off). not sure what this is about....its been a b*tch to keep the PH low enough(5.8-6.2).....i PH it, and it goes up within 24 hours.....never goes lower than i PH it for....again not sure what this is all about.....my question is this....its been 3 weeks of 24/0 on a 40w flouro.....when can i switch to my HPS lamp? Do i want to do an 18on 6 off or what? Any help appreciated.

Peace

[Elephant Man]

You want alternating nodes, the sign of a mature plant. You can switch to the hps now, but I would leave them on 24/7 till they get their second alternating node, or longer. I think I would go 1/4 strength, or maybe go back to water until they recover. I am not sure about the ph problem, I have experienced the same, anxious to see what others say.

[Biffdoggie]

I usually go at least a foot, maybe 1.5 but have done 2-3 footers before.