Metallic taste may be the chemical used to keep the salmon looking "fresh" after slaughter, keeping in storehouse, long transport from supplier to wholesaler, then to middleman, then to supermarket, long display period in the supermarket before you bought it. In these days of high oil prices, it costs a lot to keep food properly frozen/chilled.
Metallic taste is not natural, of course.
Most likely in your case, it is not Formalin because in Sing, we got strict controls.
Just to let you know, watched a TV documentary of abuse of FORMALIN (formaldehyde) used by some Indonesian hawkers/market meat sellers to keep their food stuffs looking fresh without refrigeration. Some even apply it to their noodles. Someone told me something similar about China too.
FORMALIN is used by undertakers to preserve DEAD BODIES for the wake, before cremation. It is a deadly carcinogen.
This is not fiction. This is real. So look after yourself. See:
http://nutritionfortheworld.wetpaint.com/page/The+Healthiness+of+street+foods+in+Indonesia
http://thestar.com.my/news/story.asp?file=/2012/3/24/nation/10977376&sec=nation
http://www.thedailystar.net/magazine/2007/04/01/sfeature.htm
[Quote from article] Formalin, a toxic chemical, is rampantly used to make old, rotting fish look fresh. [EndQuote]
For those who want to deride the concern over misuse of Formalin in food -
it is not funny anymore, if you get the BIG C. Most probably stomach/colorectal cancer.