if you don't have an ATO then get that before a skimmer...its so nice not having to manually top off a couple times a day plus is keeps everything more stable...
weekly 5g water changes on a 20g system should keep it pretty darn clean as long as you don't over feed like crazy or put a ton of fish in there..
if you haven't got the media baskets yet id ditch the socks and get media baskets, at least one...then you can throw filter floss on the top and bottom with chemipure blue or elite in the center. you can get filter floss for cheap in bulk, its so much easier to change filter floss than pulling out the socks 1-2 times a week and cleaning them.
another tip is when you do water changes make sure to suck out all the crap that builds up in the back sump area once a month