i agree that they are aggressive fish, but i fail to think ur tank is too small, its after all 220 litres. 1. they take years to be fully grown,2. even if they are fully grown, u are lucky to see them exceed 35cm. ur 91cm tank is sufficient. u dont need a pond to have them.
just to express something irrelevant:
samething can be said about asian aros, people often say one needs a 2 metre tank to have them, sometimes more. but the truth is in asia, it is a common practice, and commonly considered that a 1.2 meter tank is sufficient(through experience). and they seem to be happy, or at least no significantly less happy than if they are put in a much larger tank, so i dont know what is the theoritical basis for people to claim good size tanks like urs is "too small"(in ur case datnoids). if there is none, such claims would be arbitrary.