Black mollies, Acclimated over an 2-4 hours from fresh to saltwater will tear into the cyano. As the above posters posted you do have to get rid of the source though, black mollies will only hide the problem (by eating all the cyano) instead of curing the problem (most likely excess nutrients). You can usually return the mollies back to the LFS. I would probably try to acclimate them back to freshwater first, or talk to your LFS to make a plan where maybe they will keep them in saltwater and sell them for this purpose (possibly costing more than the original 1.99 you pay). But remember this isn't the cure, it will grow back if you don't fix the other problems.