How to set up an aquarium/closed ecosystem?

Hi. I am buying a 30-40 gallon fish tank and I would like to attempt to make a closed self sustainng ecosystem. My end goal is to have goldfish and guppies in there, not many, so I need brine shrimp and plankton for them to eat. Plankton are plants so they'll photosynthesize and brine shrimp can eat plankton and the fish poo. Does anyone have any tips on what else to add. I'm thinking more in terms of plants but I want everything to be self sustaining so any more bottom feeders would be appreciated. Thanks.