But I don't think keeping jellyfish in captivity is unethical. They're somewhere between a goldfish and a houseplant in terms of sentience. They have no central nervous system; just a loosely connected net of nerves that control their muscle contractions and very simple senses, like swimming towards light and migrating up in the water column at night and down during the day.

I would argue that it definitely is unethical to keep cephalapods in captivity unless they have a big tank and some sort of stimulation.