You could also get away with a pair or trio of white cloud mountain fish, especially if you have enough plants and a fluorescent light to let them grow (NOT an incandescent light, it would get too hot with a small tank like that). They now make tiny power filters for nano tanks like that, it would help with the water ecology. White clouds are hardy little minnows, very attractive and lively, and can take room temps down to at least 65 degrees. Make sure you don't overfeed, that fouls the tank quickly.

I've never tried a tank that small but I should think that the fancy tailed guppies would do all right. We've some like the yellow one in the center of this picture [ I'm not too confident in the 'female' label.] that seem quite tough and really look nice when a bit of sunlight hits the corner of the tank in the afternoon.

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What about a single lamprologus ocellatus cichlid? You can keep mating pairs in small tanks, so maybe a solo guy in a mini tank? I don't know how hardy they are though.