I did get sand in my filter but it was the really fine white stuff,and im trying to get rid of it now but the dark stuff doesnt seem to get in as its alot heavier,ive got an fx5 and xp3 by the way.
Not sure what marine sand is,its called different stuff depending on where you live.
im my humble opinion, red quartz sand would go well with black oyama on all sides of tank except front. colouration of bichir would be improved and beautiful.