We had a 3ft tank. We visited some rockpools down the Kaikoura coast and filled containers with rocks, water, and critters. Took them home, arranged all in the tank, added an AquaClear and that was it. Didn't do anything else to it. It occasionally got more sea water added to replace microscopic nibbles and we added sea lettuce every so often. We had shrimps, crabs, triplefins, starfishes of various sorts, a sea cucumber, anemones and an octopus (until he played Houdini and was found dried out on the carpet the next morning).
We had to dismantle it though after a couple of years as we had trouble keeping it cool in summer. The room sat at 28 - 32C and they need to be 15C or lower. A chiller was required but we didn't have the $1,000 to buy one. All sorts of things were tried from running water through a chillybin type chiller to dropping frozen bottles of water in it.