Please help tank going down SOUTH FAST!

[jason604]

Right after my newly purchased starfish died few days ago my corals wrnt looking so good to BAD. I did a 15% water change the next day but all my LPS were pretty much closed and zoas all closed. 3 days later my sps frags all starting to brown but most of my LPS back to normal kinda. My purple acro started to have base recession this afternoon now only couple hours later the recession seem to have doubled!!! What do I do I don't want everything dying. Here's a pic of how it was not to long ago to afternoon and now.



And here's my hammer that still on trouble.



My water parameter seems to look like its always been just my nitrate are a bit higher than normal due to my skimmer lvl a Lil too low after I removed its foam filter on the water lvl adjuster I think it still breaking in.

Phosphate 0.25. Nitrate 15, KH 7. Salinity 1.025. How can a dying starfish do so much damage. They arnt poisonous right? All effect happen right when it died. I'm planning to do another 15+gallon water change Tmr. My tank is 75g

Please help before its too late!!