There are posts elswhere about how to set up a natural cold water rockpool tank. I did one using a 3ft tank (cos I had an empty one at the time). My biggest problem was keeping it cool enough over summer. Bought a chiller recently so must set another one up.
We just went down the Kaikoura coast and gathered up rocks, sand and all sorts of greeblies and critters we found in the rockpools and dumped them in the tank. Looked good and very interesting to watch. I had an Aquaclear filter on it but nothing else. Every couple of weeks we would get a bit of sea lettuce and more sea water to top up the tank. Never fed the inhabitants much, just the occasional flake food. They seemed to live off the critters that came attached to the sea lettuce.
Our tank had chitons, anemones, sea cucumber, shrimps, rockfish, triplefins, starfish, brittlestars, hermit crabs and lots of other un-named critters.