chili corals don't survive in 99% of the closed systems anyway. what is your alk
? that's more important than calcium numbers IME.
i wouldn't go tossing corals away just yet. prepare another water change and wait and see.
tanks don't just crash. there is something that happens to cause a mass die-off.
stray voltage, a large coral or fish failing, parameter swings, especially alk
, a chemical attack (mostly leathers do this), poisoning the tank with some outside additive( what supplements/dosing?), etc.