The black spots could be from them trying to host corals. They usually develop these after being in contact with coral and those spots usually disapear with time.
the shaking is a dominance ritual. They will figure out who's going to be the female and the male (less dominant). It is totally normal and as long as they do it, they usually don't fight since the submissive fish is going to shake to the other and that seem to calm them down.
Your nitrates are way too hight and that is probably what killed your corals.
How are you mesuring your salinity? because if you are using an hydrometer your salinity could be way off and that could be why your corals died after a water change.
If you don,t have one, get a good refractometer. They cost around 25$ on ebay.
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More fish die from human stupidity than any other disease...
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