Why is my new clownfish dying? My water is tested and fine! no nitrates! what did I do?!?

So on Sunday I introduced a juvenielle clarki clownfish pair and a juvenille blue tang to my 80 gallon tank with one goby. I also added a shark egg, skunk cleaner shrimp, really small horshoe crab, and hermit since then. The clarki pair separated the first night. One slept at the bottom, the other at the top. The blue tang sleeps wedged in coral. Today, the clarkii one of the clarkiis sat behind a big live rock all day just hiding and being shy, now it is lying on the tank floor dying and breathing rapidly. Nothing has hurt it, the goby and all the other fish love it, I monitor it constantly and even set up a nighttime survellance video. WHAT DID I DO WRONG? I aculated them all really well for at least an hour and he was eating too, hours ago frozen brine shrimp I am so puzzled and dont want this part of the pair to die.