Most of the sunfish I am aware of are all ocean or estuarine fishes. Have you seen any at pet stores or you just saw them online (or somewhere else) and thought they would be a nice addition to a fish tank? Just curious.
You could probably keep a blue crab in a 20 gallon quite happily. I don't eat seafood so I'm not sure what other fish would be happily raised in a 20 gallon. Oysters also do well as long as you have the right food source and water flow. You'd have to narrow it down to specific species you wanted to keep and then I can help you find the proper research. I can't really suggest anything other than what I've mentioned above.
I usually don't get questions like this.
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Phishie

"An ounce of prevention is worth a pound of cure."

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