What is killing my bottom dwellers

Hey guys and girls looking for some help here. Seems like something is killing any bottom feeding fish I put in my reef.
150 gal combined reef system full mixed reef running for 2 years.
1 anthias
1 foxface
2 Picasso clowns
1 hippo tang
1 Kobe tang
1 potters wrasse
1 sand sifting star fish
1 cleaner shrimp
2 peppermint shrimp
50 plus snails
50 plus small hermits (1 grew to the size a quarter grey and black in colour)
1 staghorn hermit crab

I've tthe past year I've tried introducing many fish that never seem to make is passed a week. My longest survivors were a Mandarin lasted a month aswel as an orange spot jawfish
I've also tired
Clown gobie
Orange spot gobie
Diamond gobie
Twin spot gobie
Jawfish
Mandarin

I haven't seen any of them dead or being eaten they all seem fine and well eating and healthy but then just vanish after a week or so
Please help