Any fish safe rocks or wood will do, doesn't have to be any algae on it to start off.
Use old tank water as it speeds up the process, after a few days on a bright window ledge you'll see a nice green coating of algae.
Use a few bits of wood or rock, then while one's in the tank, you have others ready to swap when needed
Use old tank water as it speeds up the process, after a few days on a bright window ledge you'll see a nice green coating of algae.
Use a few bits of wood or rock, then while one's in the tank, you have others ready to swap when needed