I love the big piece of wood! It gives the "sunken tree" feel that characterizes the deep blackwater portions of the river. (The smaller one would look better at more of an angle)
Here are a few comments regarding a "strict" Rio Negro biotope. Disregard them if you're fine with a "loose" one.
Amazon swords aren't actually found in what people think of as the Amazon River. Crazy, I know.
In fact, while you might find a bit of Cabomba
, submerged plants will be very rare in this biotope. Consider some South American floating plants instead, like Salvinia
, frogbit, giant duckweed, or red root floater. Water hyacinth would also work, if you want something a bit more robust. My recommendation is to go with Salvinia and ignore submersed plants entirely. Floating plants will also be right under your lights, which means even your T5NOs will grow them nicely. Heck, double-strip T8s from Home Depot would.
Don't bother with any carpeting plants at all. You don't have the lighting for it, and they're not biotope-appropriate.
If you're cool with your giant awesome piece of driftwood being slowly nibbled away, consider a royal pleco - they chew driftwood to get at the tasty bacterial colonies inside, and they may even keep your glass clean in the process.
A school of cardinals would look fantastic in this tank. Other tetra species could work too, but cardinals are classic for a reason.
Substrate: yours will work, but some clean white river sand would be best, as the Rio Negro is known for fine white sand. Since I've just advocated removing all your substrate, I'll point out that for moving substrate out of a tank, nothing beats a small shop vac. You might add some dried oak or katappa ("indian almond") leaves to the bottom to improve the look. Skip cory cats - very few are found in blackwater habitats.
Those are my suggestions - good luck!