The lifespan pf many of the more popular fish we commonly keep in the hobby can surprise many people.  Clownfish for example can commonly live up 15 years in captivity and many tangs can live up to 25 years.

http://www.int-res.com/articles/meps/134/m134p015.pdf

http://www.advancedaquarist.com/2009/12/fish2

Conscientious Marine Aquarist, 2008, Bob Fenner