Actually just letting the cup sit in indirect sunlight for a few days with QT the marimo just fine. You can change out the water with some tank water by turkey baster, if you like. But you have the right idea. Once you get a healthy environment never introduce something with QT.
Might be getting the tank soon! I posted a "wanted" ad in the Boston section of Craigslist because I go to school in Newton on Mondays and Wednesdays and I got back a lot of replies!
One guy has a 55 gallon, with the stand and lots of accessories that he is willing to sell to me for only $50! Just waiting to see if he has any pictures and where he is located. Also gotta hope the tank fits in my car.... might have to come back for the stand on Wednesday lol.
So far she's doing good. Whenever I walk into the room, she's not clamping. As soon as I stick my face near the tank to watch her, she clamps. Will they clamp if they are nervous? I would feel much better knowing that was the reason for her clamping, but it still doesn't explain the flashing I've been seeing every so often.