I've seen in person a riverine 40 long that had about 25 Cherry Barbs, decorated with a large, long piece of driftwood and most of it coated in Java Moss. Water flow end to end was done by one really big pond size powerhead, with no filter media to speak of other than the Moss and the gravel.
Loveliest tank I've ever seen, it rivaled some of the early Amano tank images. The fish were really, really happy and in their best colors.