A watchman goby and tiger pistol shrimp would be pretty cool.
or one jawfish (either pearly, bali-tiger, or blue spotted)
or a pair of neon gobies
or one springer's dottyback
or one clown of the smaller species (ocellaris, pink skunk)
or one yellow or blue assessor basslet
or one damsel = very inexpensive I like the Allen's, and pink smith damsels

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or. . . . .