I got ya know.
I wouldn't be keeping perch if I was the Op. Your taking a wild species and would be trying to raise it in a tank that is just too small regardless of the size to begin with. The perch will grow very fast in 20l and then what are you going to do if you don't have space? Release a fish back into the wild that may now be carrying a disease and spread it too other wild fish or vice versa? Destroy the fish because you can't care for it any longer? Point being is I would stick with smaller fish; someone mentioned darters or stickleback etc which are in the perch family and are found in lots of water bodies and can be purchased online if you search enough and won't outgrow the tank you have available.