Twistersmom is correct in defining tannin and how to get it. Most of us don't want it, as the water while clean is brownish/yellowish along the lines of weak tea. Water that is slightly acidic (pH below 7.0) is acidic because it contains organic nutrients that have leeched into it from vegetation and wood, i.e., tannins and other things.
Diamond tetras (Moenkhausia pittieri) come from Venezuela and inhabit waters that are soft (hardness) and acidic. These waters are frequently stained brown with tannins, which is why they are so acidic. They do tend to really 'sparkle" in such water, but they will look very nice in clear water that is slightly acidic and soft (particularly if they are wild caught fish; commercial bred fish are a bit more tolerant), but regardless, to look their best (and they are a very pretty tetra to be sure) they need a spacious tank (for swimming) with plants at the sides and back. They should be kept in a group as they are shoaling fish by nature, and will feel secure and be at their best in colour when kept in a group of 5 or more. The males have elongated dorsal fins so its easy to tell them apart from females except in very young fish when the dorsal hasn't developed yet. The males will continually display much like the Black phantom and several other tetra species, both to rival males and to the females.