An octo will escape from your tank and might not get back in in time to not dry up. And no matter how good you think the tank is sealed, the octo will escape, right after eating all your fish and inverts.
As to the jellyfish, soryy but they just do not work in a captive tank. I looked into this in the past. Jellyfish need coldwater, 60F and below. In a tropical tank they will last for about a month before the melt away. Also they need a circular tank with powerhead creating a current all the way around. Jellyfish can't really swim liek fish and end up following a current. In a regular tank they get stuck in the corner. Plus you will need lots of plankton to feed them.