To expand on NickAu's comments -
30 litres is a small tank, it is only a couple of litres bigger than my betta's tank which has just one fish and a snail and is fully stocked.
Guppies are just about OK in a tank this size, but not males and females together as the tank would soon be very overpopulated with guppies. Even just females would result in overpopulation as they can carry sperm for several months if they've ever been in the same tank as a male.
Neon tetras, although small fish, need more swimming room than a 30 litre tank can provide, especially a circular tank which has less swimming distance than a rectangular tank of the same volume.
Mollies are big fish. Female mollies grow over 4 inches (10 cm). They need a tank at least 30 inches/80 cm long.
You have both hard water fish (mollies and guppies) and soft water fish (neon tetras). Depending on how hard your tap water is, one or other will not be happy.
As for the algae, since you have no live plants, I would have the lights on for less time, more like 6 hours a day. The lights need only be on while you are there to see the tank. So for example, if you are at work all day set the lights to come on during the evening, but they need to go off an hour before you turn the room light off to allow the fish to acclimatise to dimmer light before total darkness.