Can you actually see them moving? Since sometimes they float, sometimes they don't, I imagine that there must be a transition period in which they actually move.
Concerning names (I actually clicked on the title of your post because I was not understanding it, bit for the 5 SBD part), what about Uranium and Polonium. I can really imagine the face of your neighbours when you will let them know that you have lost your uranium and polonium somewhere in your flat :D
Note that if you choose those options, you can keep the money ;)
When I change the water they float up or stay at the ground. At every other time the movement has been too slow to observe.
I wonder how I'd lose those two :D
Don't underestimate them ;)