What is wrong with my saltwater aquarium?

i have a 30g that has been established for about 3 months, it has liverock and a deep sandbed with good powerheads and a filter for extra flow. about a month ago i started losing fish to some odd white splotchy thing. i brought a fish to the store and he said it wasnt ick(crypt) it was a bacterial infection. so i treated with this revive stuff he gave me as directed and i still lost fish. overall i have lost a lionfish, 4 clowns, a mandarin tang, a blue hippo tang, and a sailfin tang (note they were not all in at the same time THAT would be overstocking) i recently lost the blue tang because i had a blue damsel in there that was doing fine so i put the tang in and removed the damsel then the tang became effected almost immidiately. my nitrates/trites are both below 10, 0 chl , 0 ammonia, salinity is good. if this is bacteria how do i fix it? drain the tank and start new? i dont want to do that bc i have plenty of hermit , snails, a feather duster, and corals. that i would be risking. what is wrong with it. what do i do?