The only bacteria that will remove the nitrates is the anerobic bacteria, which only colonizes in the no-o2 or low-o2 areas. So how do you keep the septic tank (deep sand bed) from backing up ...? That's a great question.
1) Sand bed need to be at least 6" deep to promote anerobic bacteria.
A good starting point would be around ten times the volume of your tank (turnover rate). In a saltwater tank you want a natural flow. In nature the water on a reef is random and chaotic. So amount of flow is just the beginning.
Becareful with that Crushed Coral, a substrate that does nothing but harbor nitrates, decaying food, and waste, either have a big skimmer, turn your substrate regularly or take it out completely, and go with at least a 3mm grade sand.
Will your tank be bare-bottom or have sand ?
I would also advise against a yellow tail damselfish. They are very aggressive for their size. You could also try to get a captive-Bred ocellaris clownfish.