Do jelleyfish have much awareness of their environment? Would they care much about the size of the tank? If so, how can you tell if they think the tank is too small?
You're assuming awareness requires a nervous system. If a plant dies because of lack of sun or water, you can be sure it's aware of its environment. Conversely, even if a mammal is aware of something doesn't mean it has any meaningful effect on it.
Living things tend to be complicated systems, and assuming the nervous system the only part capable of interactions is just a bias. Lots of subsystems are a lot more complex then a primitive cortex (the immune system comes immediately in mind)
