Not sure where your from, but have you considered some north american native fish in with you goldfish and turtles? Maybe a couple sun fish and a bullhead? Then atleast water temps would be correct for all of your inhabitants at once. And beeing a cool water tank, the extra stock wont make as large an impact on water quality either.i think i've decided no oscars. Im going to have enough work with the 150 as it is. Thanks to all of you for helping me!