I will comment on the mollies since it is being "debated" in a sense. This fish, if healthy, will attain 3 inches for males and close to 5 inches (some report 6 inches) for females. A 20g or 15g is not sufficient space. Four mollies should have at the very least a 3-foot (90 cm) leng tank, and somewhere around 30 gallons/130 litres minimum. [We have all been assuming "20L" meant a 20g long tank...I hope it is not 20 litres.]
It is indeed unfortunate that so many fish stores will sell us anything we ask for. It pays to research fish before acquiring them, something we all learned by such errors; I know I did.
To the behaviour...it is possible this is a sign of trouble. Mollies are extremely sensitive fish when it comes to ammonia, and when fish prefer swimming into the filter stream it can mean something is wrong with the water, and the fish are experiencing gill issues. Can you test ammonia, nitrite and nitrate?
Byron.