Problem:
We cultivate a number of terrestrial organisms in greenhouses and stables where they live in completely artificial environments. This seems to extend far less to marine organisms, such as fish. Aquariums (especially the salt water variety) seem to be quite unstable and require a lot of maintenance, and commercial scale installations, like salmon farms, are set up in the ocean, rather than in a separate tank.
Question: Are there inherent difficulties in cultivating marine organisms, or is it just that mankind has not tried as hard as with land species? Please justify your answer.