Several thoughts.....questions. How old are these plants? Are they clones or from seed? How long have they been in the setup? Have you been checking the ph? Could be overnuting or ph...or both. 400ppm plus tap water seems a little strong to me but I have never tried clones so if they ARE ....then don't know. PH is always my first bet.

I have well water and tried a britta filter originally and was not happy with it. Leaves too much crud in your water and didn't help with my ph problems at all so ended up using RO (or distilled) water and am very glad I did.

If you have some sort of reflector with foil in it.....I could not quite follow that explanation......I would certainly get rid of the foil AND not use foil at all in your setup. Foil reflects irregularly and causes hot spots. I would use Mylar or flat white paint.