drooping Giant Vals

My giant Vals tend to grow to the top of my tank, and then
after a week or so they lay down on the lower plants. It
doesn't happen all at once, but they seem to all do it
eventually. The upper layers of my tank is sparce.. All else
seems to grow well. PH-6.8, gh-4, temp 78, 90watts full
spectrum on 40gal tall. (24"). Yeast CO2 and laterite in the
substrate. Iron unknown, KH unknown... What am I missing?
Is there something I should adjust for there liking? I'm
buying both iron and KH test kits, do they prefer high KH?
thanks for any advice,