The key to a long term marine tank is solving problems, not masking or countering the problem by adding something else. Try and understand whats happening and then you'll see the reasons for everything.
It's a delicate balance of food, lighting, flow, water quality and livestock, once you learn the basics it gets really easy. You have to get to the root of the problem or it will never go away. Jim
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It sounds like your water chemistry is way off, copper could be an issue since the inverts die off so quick. Has the tank gone thru a compete cycle?
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Salinty? How did you measure it?
RO or tap water?
List everything you've put in the tank, including additives
How much LR?
What type of substrate?
How old is the tank?
What test kits are you using?
Any external filters and how many powerheads?