Hmm. I was thinking of the ocean sunfish and some of the other smaller sunfish I learned in class. I completely forgot about the blue gill. You could probably fit maybe 2 in a 20 gallon. One of my old friends has a blue gill as a pet.
You may be able to breed blue crabs, but they have a complicated life history. The blue crab larvae morph at the opening of the estuary, and as adults occupy varying salinities of the estuaries. However, I don't think you would be able to breed the oysters. There is no way to tell what sex your oysters would be until they are ready to breed. Oysters are actually both during their lifetime. (I don't know if you knew that already).
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Phishie

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