Also with a 55 gallon your better off with a community fish tank, because you could have so many more fish. My opinion I would not do a aggresive tank without at least a 100-125 gallon becuase you can only have like 2-4 fish with anything smaller. I dont mean it is a bad thing to house aggresive fish in a 55 gallon tank but you are very limited in space. I don't know about you guys but when I fill up one tank with fish I feel the need to go out and buy a bigger one!