Do you still have the F8 with the Oscar? The F8 really needs low brackish. He will be more susceptible to disease and won't live as long in fresh water. At least this is what the experts have found to be true.
I have 2 F8s in a 30gal. They're young and pretty much stay together all the time. They're adorable.
i know you already had your question answered, im just seconding what the others said... i had one of the figure 8 puffers and they can be REALLY aggressive- and actually the figure 8's are less aggressive than the other kinds. his aggression is the reason i got him- he was bought for a large community tank and he was taking chunks out of all the other fish in the tank- some much larger than him. he was super cute so of course i brought him home. he had a 15 gal to himself for awhile but eventually i added him to my oscar tank and they were tough enough to ward him off. i had several snails in the tank that disappeared and with a little research i found they have a extremely strong bite with hard teeth and need to eat snails to control their tooth growth. depending on the breed, they require varying amounts of salt- some completely fresh, some need brackish. i do think they are extremely cute though.... id love to have another