Cyps need swimming space and the 20l doesn't provide that as Finz already pointed out.
I'm afraid if you want something natural in there, you will need a larger tank first. Even a daffodil will be cramped with 12 shellies in a 20l (ideally you'd only have 3 or 4 in there).
If you really must have something in there, dwarf praecox rainbows, more robust tetras, or a livebearer like Endler's or Some other small livebearer (regular mollies, not sailfin) will be it.
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