Oscars, Red Devils, GT, most large tropical SA/CA cichlids are pretty aggressive. With fish this big you can pretty much only fit 1 in your 55gal tank. Oscars could get up to 15'' i suppose, but I doubt it in a 55. These fish like lower Ph than your crushed coral would give them.....
There are aggressive african species and you can put more than 1 in a 55. Mbuna are aggressive but small and wont tear up a goldfish like I suppose you want. Most African Haps from lake malawi can be aggressive and a little bigger. My venustus would bully fish 3x his size until I traded him in, they get to 8'' and I fed mine live food. Super VC-10 may be hard to find but it gets as big as any of the africans and looks great.
There are other aggressive carniverous fish that aren't cichlids that someone will chime in about.
There are aggressive african species and you can put more than 1 in a 55. Mbuna are aggressive but small and wont tear up a goldfish like I suppose you want. Most African Haps from lake malawi can be aggressive and a little bigger. My venustus would bully fish 3x his size until I traded him in, they get to 8'' and I fed mine live food. Super VC-10 may be hard to find but it gets as big as any of the africans and looks great.
There are other aggressive carniverous fish that aren't cichlids that someone will chime in about.
