You could try PBW or oxy-clean. Truthfully I don't worry about it myself. I think sanitizing them with a meta solution might cause it. I can't back that up but once again I don't worry about it.
First try some soap and water, if that does'nt work, try some oxy clean. Don't fret too much at .99c for a #7 you will spend more on cleaning than buying new ones. as long as they are soft enough to seal the carboy, they are OK.