i want to make it a tropical tank with probably a mix of fish:

glowlight Danio (Danio choprae) (5)
Dwarf Neon Rainbowfish (Melanaotaenia praecox) (2)
Glowlight tetra (Hemigrammus erythrozonus) (5)
cardinal tetra (Paracheirodon axelrodi) (5)
Zebra Otocinclus (Otocinclus cocama) (4)

then a dozen of one type of shrimp or maybe 6 blue 6 red shrimp 

Blue Tiger Shrimp (Caridina cantonensis sp "blue" )      (6)
Harlequin Shrimp (Caridina cf. spongicola)
Crystal Red Shrimp (Caridina cantonensis sp. "Red" )
Grass Shrimp (Palaemonetes paludosus)(ghost shrimp)
if all else fails 
Red Cherry Shrimp (Neocaridina Heteropoda var. Red)  (6)

i want to have a planted tank but am unsure what substrate to have (i know i want color black to enhance the shirmp) but do i put down dirt then cover with gravel? or should i just use dirt? or just gravel? do i need to change the dirt ever? curently have gravel tank with plants and there growing like crazy just wondering benifits of dirt over plain gravel