PM me the forum that said it.
Common,comet,and other single tailed fish are not suited to tropical tanks, as they need the seasonal changes to regulate themselves, so they know for example when its breeding time, winter etc. To keep a common in tropical water would be to quicken its life cycle and it would grow fast as well.
Some types of twin tailed fish prefer a tropical tank such as bubble eyes and celestials as these are weaker fish.