work one problem at a time. I'd start with your nutrients. you're gonna starve your corals with those double 0's. you're gonna encounter a lot of issues with an imbalance nutrient system.
i have a few tangs in my tank that a "tang police" would hate me for, but i never fed them any nori. i do feed lrs reef frenzy once in a while and that does have some nori in it. :rolleyes:
ive had luck with copperbands. current one i have is almost a year... previous one that didnt make it when i had a power failure was 2 years. good beautiful fish to have overall if you are lucky with it.
i dont see anything bad in your pictures. whats your parameters like? have they been consistent and no large swings?
also, see any bite marks during your inspections? any pe on the current sps u have currently?
i have some greenleaf kno3 if you wanna try that. i have it as back up if my no3 ever dropped, but it never did. same link as @Mike Yasin posted for the calculations.