I've got a FOWLR mantis shrimp nano tank. I put aipstasia in it, and it looks pretty good. The aipstasia helps keep the water clean, too. I've heard that mojano anemones are similar (another tank nuisance), but they look a lot nicer.
Feather dusters and macro algaes could also help give you a reef look without having to worry about corals. These also help to filter the water.
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Double penetrated