I recommend some livebearers- guppies, or platys, or endlers! They are easy and fun to keep. You can even get some from users here; many breed them.
Other easy fish for a 40 liter are kuhli loaches, betta fish, and pygmy corydoras!
A lot of tetras are too active for a 40 liter and some are delicate.
(I'm assuming 40L is 40 liters. If it's a 40 gallon long there are TONS more awesome fish you can get!)
Welcome to AA!
10 gallon reef