Weak stems, deficiency of some type??

[sketch804]

Well I have recently started having a problem with one stem in particular, but a few that will do the same to a degree. The stem in question is Hygro. Lancea which when grown right will have a very hard stem but now my stems break easily and are very weak. I have the same problem with Rotala sp 'red' also. Everything else in my tank and even those plants look pretty healthy as far as I can see, no other deficiencies around but these I haven't the slightest clue. If you want to see current pix, check last page of my 40 breeder journal.

So my tank is as follows:
~40 breeder
~4x 54w T-5 HO lights (6700kx2, 10000k, pink plant light) plus 36" T-8 (10,000K)
~CO2 injected via in-line reactor, and I am pretty much at the threshold of how high I can go.
~I do modified EI dosing so to speak (KH2PO4, KNO3, millers microplex mixed with flourish comp. and extra iron, and I add 1 tablespoon or so of GH booster)
~PO4 is 2ppm or higher, Nitrates are 8ppm but probably a lot higher, low KH and GH.

anything else just ask, but anyone have any thoughts or solutions??

Thanks all!