You can't reasonably make a comparison from plain observation of the ocean to an aquarium environment anyway. Pollution aside, closed systems allow all sorts of things to build up.
Think of it this way... humans live in very polluted environments. If you take a human that lives in... say an industrial area that's heavily polluted and stick him in a sealed box that's a handful of times larger than he is with a basic life support system lots of things will build up in that system completely apart from the environment he was in that will affect his health... such as disease, bacteria, etc.
Anyhoo, I agree with the guys above... Jellies are nice, and many have succeeded, but it's one of those things that reaches systemal complexity that puts them in the "not worth it" category. ... JMO.