Your water is 400 ppm? Lose it and go with RO or distilled. PC is right.....you don't seem to have the idea very well of exactly what to do and so you are trying eveyrthing you can think of. Stop it.

You have regular tap water...plus CalMag.....WAY too much minerals etc in there if I am reading this correctly. Either way..(even if you don't) ..I'd dump EVERYTHING and start over. No wonder your plants look sickly.....I think you are burning them and that's why your ph won't stay down. As Stinky says...the issue could be your hydroton....but I have a feeling its much more your feeding "schedule" etc..


If you are interested I would try this.

RO water
CalMag....200
Grow nutes 200 (what kind are you using btw? A one part or three part)

and leave EVERYTHING else out of the equation. Everyone seems to think the answer is to add things....ANYTHING....instead of that perhaps they just have too MUCH junk in there already and it's causing toxicity problems. In THIS particular case I'm already POSITIVE there's junk in there.....what's the 400ppm of your water? Dirt? Sulpher? Who knows even what is IN your water so get rid of it.....and any problems it might be adding to your grow.

I'm guessing here....your info is sketchy and vague-ish....but that would certainly be my first guess in this situation.

Good Luck.