Basically, you want to cycle a tank to get the biological filtration started. Fish poop and other wastes in the tank turn into ammonia, which turns into nitrite, which turns into nitrate.
Ammonia and nitrite is toxic to fish, and even low levels can kill them. Nitrate is also toxic, but slightly higher levels can be tolerated.
Before putting fish in your tank, you want to get the ammonia, nitrite and nitrate levels to 0.
There are two man ways of cycling a tank, a fish cycle, where the ammonia comes from a fish, and a fishless cycle, where ammonia is added to get it kickstarted.
Very basically, you add a couple of hardy fish (in 50l, I'd add 1 guppy), to get the ammonia cycle started on the tank. When your levels are reading 0, you can slowly add more fish. I suggest you google 'cycling a tropical fish tank' both on this site and on normal google to find more information.
What you did for 3 weeks was a fishless cycle - not sure if you added ammonia or not?