Oysters as well as scallops, mussles, and some clams are very short lived...many less than a year and most people don't bother with the marine varieties because it can get costly to keep a constant population. That's my guess. I have seen scallops, mussles, and of course clams available for the aquarium, but never oysters. They're always in the supermarket frozen in the seafood section. Maybe they are cold water??? You have a good question there. I'm interested in reading what others might know or guess.