75g is best i got one in a 40g and its only been 6 months and he is 3/4 the width of the tank! But good thing i got a new 75g with 2 aqua 110's to help keep the water clean. If you ever see the nasty Oscar defaecate its disgusting and it goes all over and clouds the tank until the filter cleans it out. The worst if you feed lots of crickets the poo from the Oscar will be all wings and the exoskeleton of the cricket and it all floats to the top!