From what I've read, most atfs in tanks don't reach a large size because they die for various reasons before that happens. In particular, they are skittish and often end up breaking their backs or otherwise injuring themselves by colliding with the glass. They might also suffer from "sudden random death syndrome" but I'm not sure, that might just be payara. Someone correct me if I'm wrong, they may not be impossibly fragile, but are a far cry away from being what one might call "hardy."