The algae is probably a
diatom bloom and is part of cycling a new tank. It will go away in a couple of weeks.
IMO you do not want to add "some" fish and "some" snails and crabs. This will be too much bio-load for the existing bacteria and will result in an
ammonia spike. Since the
diatoms are a short term problem I would not stress over adding something to control them. They will go away on their own.
