How do I promote algae growth?

[FriendsNotFood]

Besides not having a filter and turning lights on, how can I encourage algae growth?

[Noraelle]

If you don't have anything living in them, you can add direct sunlight for a few hours a day (it increase the temperature so not a good idea if you have live things). You can also add some macroalgae (it may have algae to help seed the tank) or snails as the waste will help with the nitrogen cycle. But mostly, it's just about being patient... very very patient.

[FriendsNotFood]

What will the snails feed on if there’s currently no algae though? Or what should I feed them?

[Varanus]

You can feed a tiny amount, but mostly the snails will be able to subsist on the film of microscopic bacteria and such that develops on most any surface in the tank.