I second 6 line wrasse. They’ll fit in a 20 and stay quite busy going through the rocks.
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My firefish was quite timid and hid in a hole most of the time. Even when out, he was never very active. I like the 6-line wrasse suggestion. It a useful fish and never stops swimming. Mine swims in and out of the rocks constantly.I would personally say plus 1 to the fire fish. Always out and about in my tank. Especially nice if you go with the more exotic like the purple ones!
Lubbock’s wrasse or Midas Blenny would get my vote.