Where do I begin...?? Water change, testing for ???

Bristleworms (and there were MANY) seem to all be dying...This cannot be good. 75G tank. 1 lg Naso Tang, 1 small yellow tang, two engineer gobbies (10"), two blennies, 2 small (forget their name), 1 yellow with pink (also forget the name-small) two small damsels, 1 fire shrimp, (Think that's about all)

1 nudibranch (Need to know what to feed him!!!)

Thanks, Lois