Having just lost 3 fish, including a regal tang, you have my sympathies. Any symptoms in the fish? Gasping, color loss, eating?
How long had you had both of these fish? While cyanide collection is declining, it does still happen, and it is lethal to fish. Most cyanide fish will initially appear to thrive--healthy, good color, eating happily. Internally, their kidneys are failing, and they eventually die without showing any real signs of distress or ailment. It can take months, but all cyanide captured fish eventually die. Good stores know this and make sure their suppliers do not offer these fish, but they get through. There is one store that I will never buy fish from--they have the lowest prices around, but I've never had a fish survive from them.