dosing purple up in a bucket thats getting no sunlight wont do jack. algae
is photosynthetic: it needs sunlight to make itself. simply dosing purple up
w/ no light wont stimulate coraline growth.

sides, if its green algae, its safe so long as its not hair algae or any
nuisance algae. id add at least a floro light if you want coraline to cover
those rocks again.