HLLE [head and lateral line erosion disease, etc: tangs, particularly] dietary, possible implication of carbon use. Feed Selcon or equivalent and be sure fish gets green food.
WHAT KIND OF BACTERIUM AM I DEALING WITH?---
Fast primer on bacteria: they come in two types, gram negative and gram positive. Some antibiotics kill gram negative types, some kill gram positive types, and some [broad-spectrum antibiotics] kill BOTH.
"Your beneficial sandbed bacteria are gram negative. Cyanobacteria is gram negative. Erythromycin [ChemiClean Red Slime Remover; and Maracyn] manage to kill off cyano without killing your whole tank. But following the instructions is critical. Having a good skimmer to take the sudden dose of biomass is critical. You can still get a heavy dieoff of unwanted bacteria, that will take out your tank from sheer biomass if you have a weak skimmer.
"What sort of bacteria is, say, fin rot? Gram negative---like your sandbed. Anything that kills it will damage your sandbed. Don't, as a rule, put any antibiotic [from Greek: anti: against; bios--life] meaning bacteria killer---into your display tank.