what killed my fish and how to move forward?

[SwampyBill]

+1 on what Sk8r, Snorvich & Stolireef said...Those folks have a total of decades of experience & know their stuff. I've gained a ton of knowledge from them over the years. The rock & aragonite sand you just added probly created another mini-cycle, evidenced by the brown algae bloom. If you want longterm success in this amazing hobby, slow down, read the stickies on 'new to hobby', quit adding livestock while you figure out what's happening in your tank & learn better husbandry skills, or you'll just continue to throw money away & kill livestock.