clowns will outgrow your 125 liter so you will need a bogger tank for those, they dont grow huge but they need to be in groups of 3+ at the very least ideally you want 5+ IMO and they need the space because they are such an active fish so if you are upgrading id get a few more clowns. I dont know anything about dartfish so cant comment on them
If you want some mid level swimmers go for some tetras, i really like blind cave tetras because their abit different, a nice group of rosy tetras or neons would look nice, then a couple of mollies or something which like the top of the tank?