Java fern also fits some of your descriptions. If you put a bunch together, it might outgrow a really small tank, but it would take awhile.
Anubias nana or nana petite would also be good. You would have to attach it (and the Java fern) to wood or rocks. Cryptocorynes can live in about 1 watt per gallon or so, but once planted, they don't like to be moved.
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*Kristin*
5 Planted tanks:
55g, 40g, 29g, 20g, 5.5g
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