Hmm. I take it there's no going back to the CFLs?
I've grown duckweed in the past and liked it (most do not). But have not used it for light control. I suppose it will work if you can keep the right amount, and also keep the current from pushing it all to one side.
There's always old tricks like putting a layer or two of window screen under the bulbs.
Great to hear the "one-two" is working for you. In a few days when you have the final result, and if you have the time, I'd love it if you add a write-up of the experience to that thread.
Yeah the CFLs are long gone. Good news is my algae is going away slowly. Bad news is the excel partially melted my Val (to be expected). I won't be dosing that anymore. I'm going hiking this weekend to find some duckweed to cover up some of that light so the algae doesn't return. I will do a write up on your method this weekend!