alot to read and not much time so excuse me if i say somethin someone else already has.
first, i wouldnt add black sand. i would add red or a more natural color that will blend with the pfs. the black will give you a unnatural salt and peper look. i think if you got a red or dark brown from keller and maybe added a little black it would make your fish alot more colorful.
and if you want a natural looking background go with the color of my background, that dark dark blue. it makes all the colors really pop. but do paint its so worth it and it scrapes right off if you want to remove it (idk why you would)
i told you that filter sand would do that tho
hope this kinda helps. you can call if you have any questions you want to ask