Since you asked---my guess is you can pretty well duplicate the natural world range of blennies and gobies in a 55, and more fish in their size, like dartfish, grammas, etc in a 75. In a 6' tank, you've probably covered most of the range of a damsel or dwarf angel---
Definitely I've seen beneficial changes in fish motion by aiming two jets AT each other: chaotic water flow breaks up fish 'pacing'. I think we can learn from zoos, that went from animals in big bare square cages, to 'folded space,' created by plantings, pillars, pathways, that changed, say, a cat's route from back and forth near the bars, to a wending path of up and down and around. Much better mental health. Let's take the tack, among other things, of "What can we do to 'fold space' in a tank?" How can we improve the environment for all the fishes. And I would suggest that one neglected thing is water-as-furniture, water-as-walls, water-as-barriers, or 'sculpted water.' What can we do to make it better, how can we hyper-oxygenate the water (cheato helps), and make conditions better. Even a 6' tank can improve in this department, from a straight run to a 'folded' run.
