Wow nice! You're getting some quick growth there with the UG. Definitely faster than mine.
As for your floating problem, what are you using to plant it? I found if I didnt use the super thin pincettes it became hard to keep everything down while planting. If you are already using it, maybe break up the floating clump and plant a smaller bunch so you dont have to plant so close to another clump? Then just wait for it to fill in. Once it gets established it will send runners everywhere.