What are some aggressive fish for a 75 gallon tank?

You could have an Oscar tank containing 1 oscar (can reach over a foot) with a convict cichlid. Convicts grow to about 6 inches in length and are still tough enough if they are getting bullied by the Oscar, but generally small enough as not to be a large threat to the oscar. Silver Dollars are also a good choice to go with an Oscar. Instead of a convict you could have a Firemouth cichlid. I have kept these and they will reach about 6 inches in length, but are still big enough to hold their own against an Oscar. If you want a really agressive fish you could get a Midas cichlid. These extremely agressive fish can reach a foot in length, but you would want to give him the whole tank to himself because he would attack any other fish with him. You could try some smaller cichlids with him, but its hot or miss whether he will accept them. Another agressive fish would be the Jack Dempsey. These guys grow about 8 to 10 inches and can be agressive with other fish in the tank, the same goes with the Green Terror cichlid. Basically cichlids are the way to go with agressive fish, so do some research on any one in particular you find interesting. Good Luck!