I ran a local rockpool marine for a couple of years but lack of a chiller meant I could not keep the temperature down at the required level so had to give it up.
All I did was set up a 3ft tank, went down the Kaikoura coast and gathered up rocks, water and rock pool inhabitants then took it all home and poured it in the waiting tank (the rocks were carefully placed first ;-) )
I had a HOB filter and nothing else.
There were lots of shrimp plus little red jelly anemones, a triplefin or two and a rockfish, a sea cucumber, chitons, starfish, brittlestars, and for a while, a baby octopus that a fishing friend brought in for us.
As the room sat at 28C in summer and the water needed to be 15C or below, the freezer worked overtime freezing 3L bottles of water and still didn't get lower than 18C.