This article is from the Aquaria: Disease, Algae and Snails FAQ, by Elaine Thompson, Thomas Sasala and George Booth
Black sailfin mollies are excellent candidates for the break-in period
of a planted tank since they are cheap and easy to find. They are
usually considered expendable and are removed after a month or so. It
is important to NOT FEED THEM. If they are fed, they will not be quite
so eager to consume algae. When they are hungry, they are eager
consumers of most algae types seen during the break-in period.