Complex sentence: Many people like having more than one Arowana in the tank, that's fine and all but they must be at least 5-6 or more in the tank at a time because Arowanas are very aggressive towards each other.
Split into simpler sentences: Many people like having more than one Arowana in the tank. That is not a problem, but there must be at least 5-6 individuals per the tank because Arowanas are very aggressive towards each other.