I have a 55g community tank and I look at it sometimes and I just can't imagine putting an O in there. There's just not enough room. And as for a 30g, that's just a joke. Sure it will do ok for a while and then will die a 'mystery' death due to poor water quality way before it should. That's in the unlikely event it doesn't get HITH or something first. Honestly, if someone told me a 30 would be ok I don't think I would ever listen to a thing he said about fish again, but that's just me