Is it me or do your moss balls look really dusty lol?
As for your TDS, seems fine to me. If you dose ferts it does go up from the N in the ferts. So I wouldn't' really worry about that. As long as your gh ~5 stays that way, it'll be fine.
And finally... make some moss walls. You're not really utilizing your tank height at all (granted.. a 20L isnt' that tall).