Watch Blue Planet when they cruise the depths of those translatlantic trenches (can't think of the names), Almost every one of those critters down there would be just awesome to keep captive... of course the lack of pressure would do them in before they even made it to the surface. Below a few hundred feet is a whole 'nother world living in darkness.

Now something like a 100g tank with a few smaller squid would be really neat... I've always wanted to give an octopus tank a shot....they are WAY smart! But I'd never get any sleep with worry about there being a tiny crack he'd get out of, well that and watching all the time.