From what I know of leaf fish (which is basically nothing), smaller species would be ok in a 20g. Not sure how predatory they are though. I was told at a LFS
that they require very soft, acidic water. I was thinking about a couple myself, as they're super cool, but my water is way too hard and base.
3g betta, 29g molly, 90g Amer cichlid, 90g community, 137g African cichlid, 220g water garden, 6,500g koi pond