Um... aquariums usually have "Lucky Bamboo," not the fast growing outdoors bamboo.
"Lucky bamboo" is a type of Dracaena. It's what's sold at Petsmart.
"Bamboo," on the other hand, is a member of the grass family - and is what Pandas eat. It grows really fast and gets really tall - and probably isn't something you'd want in a fish tank.
@homegrown terror - what if you soaked and then washed the cup really well, using just lots of hot water? Would that be enough to remove any traces of soap or detergent?
@TeacherTeapot - oh yes! I use lots of PVC pipe and terracotta pots (with the backs removed or holes plugged) because they're cheap and safe! My guys prefer the straight PVC coupler pieces. They didn't like the elbow ones as much, probably because they can't see through them.