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Do you want a relatively short plant or a tall plant that isn't overpowering? With a picture one could give better suggestions.
As far as small but tall plants go, there is HM, elodea canadensis, elodea nuttallii, rotala sp. "mini," limnophila sp. 'guinea broad leaf.' To hide the filter intake, I use limnophila sessiliflora. I have the native species Callitriche verna, the plants are about a centimeter across, but the stem is easily broken and very flexible.
For a mid ground sort of plant, there is eleocharis acicularis, blyxia, rotala mini, and windlov java fern.

Do you want a relatively short plant or a tall plant that isn't overpowering? With a picture one could give better suggestions.
As far as small but tall plants go, there is HM, elodea canadensis, elodea nuttallii, rotala sp. "mini," limnophila sp. 'guinea broad leaf.' To hide the filter intake, I use limnophila sessiliflora. I have the native species Callitriche verna, the plants are about a centimeter across, but the stem is easily broken and very flexible.
For a mid ground sort of plant, there is eleocharis acicularis, blyxia, rotala mini, and windlov java fern.