I had some trouble with the ferts you are using initially too and screwed around a lot before finding a balance point. You are pre-mixing with water right? just checking.
Another possibility is that those stems are depleted so you need to take top cuttings and replant. This is typically needed with ludwigias and though I have no experience with these plants in particular it is a common thing that you need to do with stem plants.
If you have green water problems perhaps instead of dosing liquid work with undergravel fert tabs until growth is rapid enough to uptake the nutrients fast enough.
regarding the light issue:
Amano believes that wpg is a bad measure of actual light energy that plants receive and therefore uses something like lux/sq in or something. He uses a TON of CO2, ferts and light to promote extremely rapid growth that without expert handling quickly goes out of balance. Most of us would do just as well, as sploke suggests, with less.
His methods would also dictate fewer wpg as tank size increases being that sq. in. of surface area increases at a slower rate than total volume. After 24" of depth you have to switch to halides though.
just my 2 cents