I'll have to look for those cooler ballasts sometime.
I only had 2 watts per gallon. The 80 watts was on the 40 gallon.
The floating plant that was in the discus tank was water lettuce.
Sounds like you might be watering it too much. I water my neps twice a week. They like to be a little moist, not soggy.
It should be fine with diffused light.
I believe I've read people using the same mix as you, orchid bark w/charcoal. I have had great success with equal parts peat moss, LFS, and perlite. I thought about trying some other mixes but thought: why fix it, if it ain't broke.