perhaps heavily plant with tall plants like dwarf sag, a sword, taller crypts, jungle val, lilies, act. Anything that can reach the surface and droop over to create that enclosed feel. Then a single species of tetra? this way you can add a slightly larger bioload as its only 1 species and there are plenty of plants.
maybe 8-10 medium size tetras
or 10-12 embers, neons, cardinals, or blacks