Complex sentence: These aggressive fish are not recommended for the standard set-up; their requirements and disposition mean they're best kept alone or with very peaceful species, since much bigger or more vigorous fishes are likely to intimidate and outcompete it.
Split into simpler sentences: These aggressive fish are not recommended for the standard set-up for reasons already touched upon. It's requirements and disposition mean it's best kept alone or with very peaceful species since much bigger or more vigorous fishes are likely to intimidate and outcompete it.