Thanks for the replies! The Stingray light I keep seeing mentioned isn't as expensive as I thought, so I might splurge on that instead, which I think will improve my chances of keeping plants alive. Right now I'm set up in a dorm so I don't want to get too fancy because I'll have to tear everything down at the end of the semester anyway, but if I can keep a plant or two going until then I will probably upgrade my level of care for them.
You really killed duck weed???
I think you need to get a hold of RussellTheShihTzu She has never been able to get rid of that stuff for years. I think she'd like to know how you did it.
I really did and I wish I knew how I managed it! The second time around my goldfish ate some, but even the stuff I had in a separate tank just wilted away, despite being next to a window and under a supplemental light.