I don't plan on putting too much else in here.. a few marble hatchet fish and a couple aqua rainbow fish and I'll be done.
I'm not sure what you mean by "aqua" rainbowfish, but if you're thinking of
Melanotaenia lacustris or turquoise rainbowfish, you may want to consider another species. They get too big for a 36--about five inches long--and like all schooling fish, they really need to be in a school of at least 6 to be really happy and show their best colors and behavior. They also need drastically different conditions from hatchetfish--
M. lacustris do best in cooler (about 72F), slightly alkaline, slightly hard water, whereas hatchets, which are delicate, really need warmer (about 78F), slightly acidic, softer water in order to thrive. The rainbows would also probably be too boisterous for the hatchets, which are high-strung, fragile little fish--the hatchets might be prone to leap out of the tank if they get too rattled.
I'd just get some more of the tetra you already have--6 to 12, depending on the species and size. You'll see more interesting behavior if you have a school instead of just one or two, and they'll be much more compatible, in terms of water parameters, with what you already have.
If you have your heart set on rainbowfishes, praecox or "dwarf neon" rainbows,
Melanotaenia praecox. would be a better bet. They're a beautiful sky-blue with red fins, so you still have a lovely fish, they stay smaller (about 2.5-3"), and they do well in slightly soft, neutral to slightly acid water. And unlike
lacustris--although they're also generally very peaceful--they won't view a very small tetra as a potential meal when they reach full size.
