Regular oysters for eating usually grow in a very silty and muddy environment and are filter feeders. They likely will not survive in your aquarium for very long because of the lack of nutirents in the water column. I have heard of a special much smaller kind of oyster that was more suitable for the aquarium but I can't remember the link where I saw it at. If I happen to come across it again I'll post it for you.
-RJ
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ASCOT (MOTORCYCLE)