I had a rock pool marine set up for a couple of years and it was a great little tank. It had a HOB filter but no skimmer or anything else. I had a problem with heat though as it got too hot over summer months so I definitely recommend a chiller. Wouldn't go smaller than the 3ft either I don't think. I had anemones, chitons, rockfish, triplefins, hermit crab, sea cucumber, shrimps and, for a while, a baby octopus.