I'd say the easiest way is to have direct sunlight..but why would you get ottos if you don't have algae. If you ever do start getting algae..that'd be the time to get ottos. Not get them, and then try to get algae.
you can buy algae tablets to feed them. You could try putting some smooth stones or pebbles in a bucket in the garden in full sunlight, but you would need to keep swapping pebbles as the ottos cleaned them.