Coldwater rockpool marines are easy if you keep the temp 18C or less. Pref 15C.
I set mine up using a 3ft tank and an AquaClear HOB filter. Just took several large containers down the Kaikoura coast and gathered rocks, sand and any critters I could find. This included red bead anemones, sea cucumbers, brittle stars, cushion stars, starfish of other varieties, chitons, hermit crab, rockfish, triplefins and lots of shrimp. We also had a baby octopus for a while (little escape artist got out though and fluffed to death on the carpet).
Every now and then we would top it up with fresh sea water and drop in some sea lettuce for food. Otherwise we didn't do anything else to it at all.
Summer and the room temp sat at 28+ and we were having to drop frozen 3L bottles of water in there to get the temp down. Too much effort. The inhabitants can take high temps for short periods of time (as the tide goes out) but not long term.
Have since bought a chiller but not got around to setting up the tank again yet.