lost powder blue...but why?

[Rondelet]

If your fish was doing fine and eating well for several months, I would doubt a sudden downward spiral was due to cyanide. Also, I don't think PBT are poor choices. Many aquarists keep them quite successfully.

Were there any signs of disease on the fish - and in particular lacerations, cuts, or haemorrhages? Tangs, and PBTs in particular, can be quite aggressive and scrap a lot. Often such behaviour is delayed and can appear if a fish gets larger and suddenly challenges the pecking order status quo in the tank. If your PBT got into a fight and was wounded, it's very possible that a septicaemia (basically and internal bacterial infection) caused it to go downhill very quickly and die. Just a guess of course. Other issues such as nutritional deficiencies are usually chronic and progress slowly. A toxin or poor water quality, which can cause your fish to go down fast, would normally affect more than one fish. So I would probably rule both of these out, leading me to suspect an internal bacterial infection.