Why do only some of my saltwater fish die off?

I have a 150 gal tank. PH is 8, nitrate is 20ppm, nitrite and ammonium are 0. I have two types of shrimp, a sand sifting and a serpent star. Along with 4 different snails and 3 different . All are living creat along with the four different corals I have. I have added fish and some live and some die. My clownfish, Cardinal fish as well as my blet are living well, but I have lost a Jawfish, a foxface, two blennys and a copperbanded erfly (I do have plenty of aiptasia in the tank for it to feed on). I cannot figure ot why these specific fish are dying and why some are not. The lighting is 3 250W MH along with PC and moonlights at night. The temp is between 76 and 81. There is a 55gal refugium. I also have a fluval 404 on the system. In addition, the tank has 100lbs of crushed coral, 200lbs of live sand, 250lbs of live rock and a proteint skimmer. The Ref has 90lbs of fiji mud, 50 lbs of curshed oyster, 50 lbs live rock and a protein skimmer. Any help would be great