I have dwarf lettuce and creeping jenny floating around, which I don't pen up, but I do pen up the duckweed since if I don't it tends to die off after hitting the filter current and then it clogs the filter, but my tanks usually have a much higer flow than normal tanks so you might not need or pen yours up. It will increase light to the bottom plants but as long as enough light gets though you'll be fine. As it stands about half my tank is shaded with duckweed/creeping jenny/dwarf lettuce/floating anubias and I still get crazy fast growth rates even with relatively low light.
I've heard of large-leafed duckweed but I've never seen it in person or seen a photo of it before. Duckweed will grow to different sizes based on lighting and nutrients. For example, I put a handful in an indoor pond, and a handful to an outdoor pond. The duckweed indoors with less intense lighting grew larger and greener with longer roots. The duckweed outside in full sun gew very small, light green with shallow roots, but it multiplied MUCH faster than the one indoors.
I don't think duckweed's roots get very long, ever, so if it's got long roots (over 1/2), it's probably some other floater like frogbit or salvinia.
Duckweed is my primary floater. Is it normal for duckweed to make larger versions of itself, or is this one of my plants that I threw in the tank and forgot about? I know, it sounds horrible. I will get pics tomorrow. I have a plant that I've sort of let float and it has SUPER long roots. Like, almost halfway down the tank long. Not sure if THAT is frogbit or not.
I'm quite ashamed, but my girls love all of the random tanks floating around. I've been working on my boys' tanks and saving the sorority's for last since I'm moving and carrying a 20 gal up two flights of stairs is exhausting in itself.
What kind of floaters and how many do you have? How many and what type of plants do you plan to have in the rest of the tank?
If you only have a few floaters and low-med light plants, you can go with a 30-40w fluorescent tube. You can also use two 20w compact florescent inside of clamp lamps, one on each side, crossing in the middle. The color temp of 6500k is most like daylight in terms of color appearance.
If you have a lot of floaters (half the tank or so), I'd use a little airline tubing glued to suction cups to corral the floaters into one or two areas and leave the middle open.
What type of bulb should I get for my 20H? I'm going to be switching to dirt+sand substrate soon and I'd really like to not have to worry about the light getting past my pesky floaters. Currently I have the lighting that came with my tank's hood. Some plants are growing fine and some not so much (as to be expected) right now. I'd like to get appropriate lighting for the bottom plants to flourish. (: