With RO water you are looking at soft and very soft water fish species. Angelfish and gourami fit this, but the tank size now comes into play; there are gourami that would be suited, provided other fish in he tank are compatible. Not angelfish here.
Angelfish are shoaling fish that need a group of their own. They cannot be kept as two fish (unless they are a bonded mated pair), three fish, or four fish. A single angelfish I do not recommend because this is denying the fish something it considers necessary, and that is not the way to maintain living creatures. Back to the group of five or more...as they mature they will get large, around 6 inches/15 cm in length, with a vertical fin span of 8 inches/20cm. They will develop an hierarchy within the group, hopefully but this depends upon the individual fish. Males are territorial, and a dominant male will exert his right to his territory, and in this sized tank that will be the entire tank space. Other males will be seriously harassed and possibly killed. The female he selects will often be the only female tolerated in his space.
Gourami offer several options. The other fish must be very peaceful, not likely to fin nip, and not active swimmers. Gourami are quiet, sedate fish. Some species are reasonably peaceful, some are anything but; males are territorial, but the degree to which this plays out depends upon the species and then individual fish. We can go into specifics beyond these generalities but only for a particular species. One I would suggest you avoid is Trichopodus trichopterus, which has several man-made varieties--blue, gold, 3-spot, cosby, marble, opaline, and probably others I cannot remember--but they are the same species with the same inherent traits and can be extremely aggressive not only to each other but sometimes every fish in the tank.
Some of the dwarf cichlids could work here, but not with gourami, it is either or. There are some slightly larger tetras that make nice centrepiece fish, thinking of the Bleeding Heart Tetra, in a group of 9 minimum.